Numbers 3:5-7, “And the Lord spoke to Moses saying, ‘Bring the tribe of Levi near, and set them before Aaron the priest, that they may minister to him. They shall keep guard over him and over the whole congregation before the tent of meeting, as they minister at the tabernacle. They shall guard all the furnishings of the tent of meeting, and keep guard over the people of Israel as they minister at the tabernacle.'” ESV
The Levites served as “Honor Guards” for Aaron, his sons, and Moses. Their responsibilities were similar to the “Secret Service.” They guarded every item making up the tabernacle, and the priests.
They guarded the people by keeping them away from the holy things. If the people even looked at the holy things they would die.