The phrase “tamim haya be-dorotav” means that Noach was “humble in his generations.” That is to say, despite his piety, and the vast difference in moral and spiritual stature between him and his contemporaries, Noach remained humble and self-effacing. He did not pride himself for his righteousness or look condescendingly upon others.
The Torah in the beginning of Parashat Noach describes Noach as an “ish tzadik tamim…be-dorotav” – a righteous and “wholesome” person “in his generations.” Rashi famously cites two views concerning the word “be-dorotav” (“in his generations”), as to whether it emphasizes the fact that Noach was righteous despite the influence of his corrupt contemporaries, or notes that Noach was righteous only in comparison to his generation. Had he lived among more pious people, according to this second view, he would not have stood out as an especially righteous individual.
There might also be another approach to the word “be-dorotav” (as suggested by Rav Chaim Braun, in his Toledot Chaim). According to the cantillation notes, this word is linked specifically to the word “tamim,” and not with the description of “ish tzadik.” The more correct rendition of this verse would be, “Noach was a righteous person; he was tamim in his generations.” Now Rashi, in his commentary to Masekhet Avoda Zara (6b), interprets the term “tamim” in this verse to mean “anav u-shfal ru’ach” – “humble and meek in spirit.” If so, then the phrase “tamim haya be-dorotav” means that Noach was “humble in his generations.” That is to say, despite his piety, and the vast difference in moral and spiritual stature between him and his contemporaries, Noach remained humble and self-effacing. He did not pride himself for his righteousness or look condescendingly upon others. He was “humble and meek of spirit” even while living “in his generations,” among people who were very far from his moral and religious stature.
It is tempting to look around and take note of unimpressive people in order to feel accomplished and proud. The phrase “tamim haya be-dorotav” might teach us to evaluate ourselves honestly by weighing our achievements against our potential, rather than against the mediocre or poor standards followed by others. We should not be patting ourselves on the back for rising above the low standards we see around us, and should instead humbly acknowledge our shortcomings and be constantly working to grow and improve.