You fill it with construction material to make stuff.
things you can make:
-healing tent, use it to restore health.
-catapult, can be driven and shot, rather buggy though.
-small siege tower, bugged, doesn't spawn at all.
-large siege tower, spawns a siege tower that can be driven around.
-weapon rack, bugged, doesn't spawn at all.
each needs a different amount of material from 5 for the healing tent to 85 (i think) for the large siege tower.