Which Ma'aser is this Pasuk referring to?
Rashi and Ramban: It is referring to Ma'aser Sheini which must be eaten within the walls of Yerushalayim. 1
Seforno: It is referring to Ma'asering crops and animals, because giving Ma'aser causes an increase in one's crops and in one's flocks. 2
Rashi: It cannot be referring to Ma'aser Rishon, which the Torah already discussed in Korach, Bamidbar, 18:26, and which can be eaten anywhere (since it is Chulin which does anyway not need redemption - Ramban) - as the Torah stated there in Pasuk 31. (Nor can the Torah be referring to Ma'aser Ani, which the Pasuk discusses in Pasuk 28).
How will eating one's Ma'aser Sheini in Yerushalayim teach a person Yir'as Shamayim?
Ramban: The Kohanim and the judges who stay in Yerushalayim will teach him how to fear Hashem, in addition to teaching him Torah and Mitzvos.
Rashbam: 'When you see the location of the Shechinah - the Kohanim performing the Avodah, the Levi'im on the Duchan (singinf) and Yisrael in their places (standing by their Korban)'.
Seforno: Because in the place that is chosen for the Beis-Hamikdash, that is where the Beis-Din ha'Gadol sit to teach and to Pasken.
Why does the Torah insert B'chor Beheinah in the Pasuk of Ma'aser Sheini?