When Aaron and his sons have finished covering the holy objects and all their equipment, as soon as the camp is ready to move, the Kohathites shall come and do the carrying. But they must not touch the holy objects, or they will die. These are the transportation duties of the Kohathites regarding the Tent of Meeting.
We have here a second muster of the tribe of Levi. As that tribe was taken out of all Israel to be God's peculiar, so the middle-aged men of that tribe were taken from among the rest to be actually employed in the service of the tabernacle. Now observe, I. Who were to be taken into this number. All the males from thirty years old to fifty.
Commenting on Numbers 4:1-20
And to the office of Eleazar, the son of Aaron the priest, pertaineth the oil for the light,.... The following things, to bear them, according to some Jewish writers; but rather it was to give the oil for the light, that is, the vessels, and so the vessels of the oil to the sons of the Kohathites; agreeably to which are the Targums of Onkelos...
the sons of Kohath shall come to bear it, but they shall not touch any holy thing, lest they die--The mode of transport was upon the shoulders of the Levites (see on Num 7:9), although afterwards wheeled vehicles were employed (Sa2 6:3; Ch1 15:12).