# the tabernacle of the covenant decrees

The tabernacle was also called by this longer name because the ark with the law of God was placed inside it.

# everything in it

Here "it" refers to the tabernacle.

# The Levites must carry the tabernacle

"When you travel, the Levites must carry the tabernacle" 

# make their camp around it

"set up their tents around it"