What exactly, is Hashem telling Moshe in this Pasuk and in the next one?
Rashi: He is telling him that having appeared to the Avos with the Name Keil Shakai and made a covenant with them to save their children from the slavery and to take them to Eretz Cana'an, He was now obligated to fulfill it. 1
Seforno: Besides redeeming Yisrael in order to establish them as His people, Hashem also needed to do so to fulfill the covenant that He has made with the Avos, and because He had heard Yisrael's cries and prayers from their troubles. 2
Why must the Kohen who removes the ashes from the Mizbe'ach change his clothes?
Ramban: The Kohanim had two sets of clothing, one better quality (newer?), the other, inferior quality - and it is a Mitzvah when taking out the ashes to change into the latter (for the reason cited by Rashi).
Which ashes is the Pasuk referring to?
Rashi: It is the ashes that were piled on the 'Tapu'ach' (the pile of spent ashes that had been removed from the Ma'arachah). 1
When it became excessive (Rashi).
To where did he take the ashes?
Rashi: Refer to 4:12:2:1.
Why does the Torah find it necessary to add "el Makom Tahor"?
Rashi: Refer to 4:12:3:1.