What did the bull come to atone for?
Rashi #1: It came to atone for all Zarus 1 and abominations that were (or would be) performed on it.
Rashi #2 (citing the Toras Kohanim): It atoned for any stolen items that may have been donated regarding the work of the Mishkan and the Mizbe'ach.
Ramban (citing the Ibn Ezra): It came to atone for Aharon and his sons together with the two rams. 2
Seeing as both the two rams together with the loaves, and the bull were brought each day, why does the Torah split them into two, the former in Pasuk 35, the latter, in the current Pasuk?
Rashi: Because the previous Pasuk discussed bringing the rams that atoned for Aharon and his sons seven consecutive days, but not the bull, which was brought in order to atone for the Mizbe'ach.
Oznayim la'Torah: Because, whereas the two rams were an Olah and a Shelamim, which can be brought voluntarily, the bull was a Chatas, which can only be brought obligatorily. Consequently, the Torah needs to mention it separately, in order to inform us that, although they had already brought a bull on the first day, they were obligated to bring it again on each of the seven days - since the atonement required seven days in order to be effective.
What does "ve'Chiteisa es ha'Mizbe'ach" mean?
Rashi: It means that Moshe was to place the blood of the bull on the Mizbe'ach with his finger.