# When the camp prepares

"When the people prepare". Here "camp" refers to all of the people in the camp. 

# to move forward

"to move to another location" 

# the screening curtain

This refers to the curtain that kept people from being able to see into the most holy place.

# cover the ark of the testimony with it

The word "it" refers to the curtain that separated the most holy place from the holy place.