Charles Ellicott Commentary Numbers 3:38

"And those that encamp before the tabernacle eastward, before the tent of meeting toward the sunrising, shall be Moses, and Aaron and his sons, keeping the charge of the sanctuary for the charge of the children of Israel; and the stranger that cometh nigh shall be put to death." — Numbers 3:38 (ASV)

Keeping the charge of the sanctuary ... — The word mikdash (sanctuary) appears to have a more comprehensive significance than mishkan, the acacia wood building, or ohel, the tent that covered it. It seems to include the court that surrounded the Tabernacle, as seen, for example, in Leviticus 12:4 and Leviticus 21:12.

For the charge of the children of Israel — that is, to attend to everything that was commanded the children of Israel.