מִבֶּ֨ן שְׁלֹשִׁ֤ים שָׁנָה֙ וָמַ֔עְלָה וְעַ֖ד בֶּן־חֲמִשִּׁ֣ים שָׁנָ֑ה כָּל־הַבָּ֗א לַעֲבֹ֨ד עֲבֹדַ֧ת עֲבֹדָ֛ה וַעֲבֹדַ֥ת מַשָּׂ֖א בְּאֹ֥הֶל מוֹעֵֽד׃
from thirty years old and upward even unto fifty years old, every one that entered in to do the work of service, and the work of bearing burdens in the tent of meeting,
Rashi on Numbers
עבדת עבדה THE SERVICE OF THE MINISTRY — (lit., a service of a service). This is the song of the Levites with accompaniment of cymbals and harps, this being the service attached to another service (the sacrificial service) (Arakhin 11a).
Rabbeinu Bahya
Siftei Chakhamim
Service for [another] service. Meaning that at the time when the Kohanim were making offerings and dealing with the service, the Levites were singing and playing on their cymbals and harps. Thus they performed a service for the service of the Kohanim.