They, i.e. the priests and Levites. Before thee, i.e. upon thine altar of incense, which stood before the ark, the place of God's special presence.
Related Commentaries of Deuteronomy 33:10
Deuteronomy 33:10
10 They shall teachb Jacob thy judgments, and Israel thy law: they shall put incense before thee, and whole burnt sacrifice upon thine altar.