Sefer Devarim begins with Moshe's speech which is a "Biur" – an explanation - of the Torah. It is logical to assume that a gap exists between the text itself and its explanation, and they are not identical. The explanation is a layer of text which is later than the text it comes to explain.
Due to the ramifications of this assumption, it is possible that the Sifri minimized the meaning of the word "Biur", rather positing that Sefer Devarim is merely a repetition of what was already said in the Torah and not an explanation. "He who learned a verse and forgot it, should repeat it… Therefore Moshe explained the Torah."
The Sifri's approach is off the mark with regard to the meaning of the word "Biur". However, the advantage of the Sifri's approach is in the fact that it removes any questions as to the exceptional nature of Sefer Devarim with regard to the other Chumashim of the Torah. Is Sefer Devarim made out of the same material as the other Chumashim? Are the words of Sefer Devarim God's words or Moshe's? The Sifri's approach was adopted by the Ramban who suggests that all of the content of Sefer Devarim was already said by God. Even those new Mitzvot that were not previously mentioned in the Torah, were previously told to Moshe at Sinai and were not introduced in Moshe's speech in Sefer Devarim.
However, the majority of commentaries understood that "Biur" is an actual explanation addressing the needs of the students learning the text. This need stems from the existence of a gap between the text and the learners of the text. The existence of this gap might be explained in a variety of ways. The simplest explanation of a gap can be a deficiency in understanding the language. The language of the original text is not the language of the student who needs a translation, which by definition is also an explanation. A good translation incorporates not only the lexical meaning of the original words, but it also expresses the feeling, environment, and the precise context of the text.
According to Rashi (based on Midrash Tanhuma), Moshe's explanation was indeed a translation: "He explained it to them in seventy languages." While it is unclear which of these seventy languages were necessary to assist the Nation's understanding of the text at that point in time, Rashi presents us with an easy and simple explanation as to the need for an explanation. While Rashi has reduced our wonder as to why the Torah has to offer an explanation of itself, his explanation does not seem to be consistent with reality. It seems unlikely that the nation that Moshe was speaking to was so multi-cultural and multi-lingual that it needed a translation into seventy languages. Therefore other commentaries chose alternate explanations. However, one could try to understand Rashi in a non-technical vein. Moshe was attempting to express the seventy facets of the Torah. Moshe attempted to deepen the understanding of the Torah through the use of translation. The act of translation reveals a variety of exegetical possibilities.
Rabbi Hirsch took a completely different approach. "Biur" does not refer to an explanation, but the revelation of the underpinnings of the Torah's worldview that is the basis for our obligation to the world of Torah and Mitzvot:
"These speeches, written in the first eleven chapters, present the basic viewpoint and general obligations towards God and his Torah… And each Jew must internalize them in order to be faithful to God's covenant. These ideas include fear of God, love of God and attachment to God, that are all based on our theoretical and practical knowledge of God's unity. Ultimately, this refers to the Mitzva of learning the Torah that each Jew is obligated to partake in."
According to Rabbi Hirsch, Moshe dedicated his final days to the values of tradition. He is not dealing with philological, textual or linguistic explanations. Rather, he is building the necessary infrastructure for every Jew – the worldview of the Torah. Moshe is attempting to reveal deep secrets in order to cause each Jew to understand the meaning of the Mitzvot, the reasons behind them and ultimately to connect to their worshipp of God.
Rabbeinu Behayey presents a different, Midrashic interpretation: "The Torah can be learned with 49 explanations to purify and 49 explanations to defile." According to this interpretation, in Devarim, Moshe moved to a new medium, different than the one he used throughout the years in the desert. In his last days Moshe initiates the institution of the Oral Law and begins explaining the Torah in this way.
Rabbeinu Behayey's approach is attempting to obscure the difficulty by claiming that Moshe's explanation is a non-Peshat explanation. If Moshe is introducing the Oral Law into the Written one, we are exempt from dealing with the theological questions of the need for an explanation of the Torah.
However, Rabbi J.B. Soliveitchik dealt with the question head-on and suggested a solution:
"The name 'Mishneh Torah' teaches us that Sefer Devarim is in fact the Oral Law. The Oral Law comes to explain the Written Torah and that is what Sefer Devarim is doing. Therefore, in addition to being considered the Written Torah like the other four books, Sefer Devarim is also considered Oral Law, thus making this book the most unique of the Five Books of Moshe. Thus, the first four books were said by God as the Written Torah, while 'Mishneh Torah' is different, because it was said by God to Moshe both as the Written Torah and as the Oral Law." (Reshimot Shiurim Yevamot 4B)
Rabbi Soliveitchik redefines the essence of Sefer Devarim as a type of learning that belongs to the Oral Law! Its source remains the Written Torah that God handed to Moshe and Moshe to the people. The tremendous advantage of this position is the connecting of the methodology of the Oral Law into the Written Torah. It connects what is written to its explanation, what was written to what was transferred orally, the Peshat to the Derash and unifies it all under the name of God's Torah. This truth exists not only in the Halakhic world of obligations, but also in a theory as well. This viewpoint upgrades the Talmudist learning to an essential, hidden part of the Written Torah.
At the foundation of this approach offered by Rav Soliveitchik is the practical assumption that at the end of his life, Moshe himself spent time delving into the Oral Law as part of the Written Torah.
Rabbi Soliveitchik's uncle, Rabbi Y.Z. Soliveitchik, presented a slightly different formulation:
"With regard to Moshe's obligation to transfer the Torah to Bnei Yisrael, in addition to his obligation to transfer the actual words and prophecies that were said to him, he also had to explain the meaning of the words. This second obligation was not fulfilled as a prophet, but as a teacher who is obligated to teach Torah. All of this together is included in Moshe's obligation to pass on the Torah to the Nation." (Hidushei HaGriz 142)
According to his approach, Moshe wore two different hats. Moshe was the prophet who presented the people with actual content of the Torah. In addition, Moshe was the teacher of the Torah. In this second position, Moshe was responsible not only to say the verses of the Torah, but to explain them as well.
In conclusion, Sefer Devarim presents to us a new Moshe. In order to understand the greatness of this character that we view in the final days of his life, we must understand the meaning of the word "Biur" - explanation. Explanation demands interaction. It demands a clear understanding of the difficulties and needs of the person standing before you. It demands leaving one's areas of comfortability and refraining from dealing with words and texts. Every human interaction demands an act of explanation. In every interaction, one must understand and explain to one's self the essence of the interaction and explain what was seen and heard. Therefore, at the beginning of Sefer Devarim, right before his death, we encounter Moshe's first real interaction with the Nation, in the fullest meaning of what an interaction is. Until now Moshe 'said.' Now Moshe is explaining. Perhaps at the end of his life, facing his humanity, Moshe succeeds in connecting with the nation that he led for so long and sees the world from their point of view. He succeeds not only in 'saying' the Torah, but giving over the meaning. He reads the Torah from the viewpoint of a receiver of the Torah and not only from the viewpoint of the giver of Torah. And in this he transforms from a giver into a teacher.