Albert Barnes Commentary Numbers 18:4

"And they shall be joined unto thee, and keep the charge of the tent of meeting, for all the service of the Tent: and a stranger shall not come nigh unto you." — Numbers 18:4 (ASV)

A stranger – that is, everyone not a Levite. So in (Numbers 18:7), it denotes everyone who was not a priest: compare (Numbers 3:10); (Numbers 16:40).