1)
Which service is the Pasuk referring to?
1.
Rashi and Seforno: It is referring to guarding the Mishkan, the onus of which lay on the shoulders of Aharon, 1 and whom the Levi'im were now appointed to assist, to make sure that no Zar enters. 2
2.
Oznayim la'Torah: It refers to the Avodah os singing, which the Levi'im performed whilst the Kohanim brought the Korb'nos Tzibur.
2)
Why did the Mishkan need to be guarded?