Rashi: It means 'lowly (humble) and patient (long-suffering)'. ¹

Oznayim la'Torah: Because Moshe, attributed his high spiritual level to the facts that he merited to ascend to the Heaven at Har Sinai, and that he was able to speak to Hashem whenever he wanted; and he figured that, if anybody who lived 'on the ground' would have those merits, he would attain a far higher level of spiriruality than he had. ¹

Sifri: To teac us that he may have been a greater Anav than any human-being, but not greater than the angels. ²

Ramban #1: To teach us that Hashem took Moshe's part precisely due to his humility, since Moshe would never retaliate, even if he knew that someone had said something offensive about him.

Ramban #2 (citing the Ibn Ezra): Moshe would never seek to be superior to anybody else, nor would he flaunt his superior position to anybody else - certainly not to his siblings.

Da'as Zekenim and Hadar Zekenim: To demonstrate that he did not marry the queen of Kush due to haughtiness.

Targum Yonasan: Because that explains why he completely ignored the Lashon ha'Ra that they spoke about him.

Nedarim, 38a: Because Anavah is one of the fouir things that a person requires in order to merit Nevu'ah - one of them is Anavah, ¹ and this is the source that teaches us that Moshe was an Anav.

Rashi (in Sefer ha'Pardes): It can be read 'Anu'. In answer to their criticism that, on his own initiative, Moshe had separated from his wife and had negated the Mitzvah of 'P'ru u'Revu', Hashem challenged them 'Anu Bi!' Testify against Me, when did Moshe ever do anything against My wishes?

Da'as Zekenim: When you write out the letters 'Ayin' (Ayin, Yud, Nun), 'Nun' (Nun, Vov, Nun) and 'Vov' (Vov Vov), the Gematriya is two hundred and forty-eight - to teach us that he was humble in all his limbs.

Rosh #1: Moshe omitted the 'Yud' because it was hard for him to write such praise about himself. So Hashem said to him, 'Since you omitted a 'Yud' to detract from your honor, add the word "Me'od", to increase it!' ¹

Rosh #2: It is because Moshe omitted a 'Yud' from Hashem's honor when he said "Notzi lachem Mayim" ('shall we bring you water ... )?' and not 'Yotzi lachen Mayim' ('shall He bring you water)?'. ²

Or ha'Chayim (Sh'mos 34:29): Due to Moshe's humility, he did not want to write that he was "Anav Me'od mi'Kol Adam" (Bamidbar 12:3). Hashem allowed him to omit the Yud in "Anav". Due to this, a little ink remained on the quill, from which he merited that his face radiated.

Ba'al ha'Turim: Because without the 'Yud', the Gematriya of "Anav' is a hundred and twenty-six ³ - equivalent to the number of days that Moshe spent on Har Sinai.

Lev Eliyahu (Bereishis p.125): Just like pride is the ultimate evil, so is humility is the ultimate good.