Numbers 18:3
And they shall keep thy charge, and the charge of all the tabernacle: only they shall not come nigh the vessels of the sanctuary and the altar, that neither they, nor ye also, die. And they shall keep thy charge, and the charge of all the tabernacle: only they shall not come nigh the vessels of the sanctuary and the altar, that neither they, nor ye also, die.
The Levites were assigned to assist the priests and to keep charge of the tabernacle and all its furnishings. Aaron and his sons alone were permitted to minister before the Lord in the priestly office. The Levites were forbidden to come near the vessels of the sanctuary and the altar to perform priestly duties. They were not to see the holy things until those things had been covered.
The boundary preserved the distinct duties God had appointed for sanctuary service. Violating that boundary brought death. The danger extended to both the Levites and the priests, so that neither they nor the priests would die only if the separation was kept.