Care to explain? And your pfp is fucking adorable!
According to the wiki it is there for players to catch anything that clipped through the floor of the base. There are actual barriers in place to prevent the player from truly falling out of bounds.
Gotta say the collisions has been super reliable in this game. Can recall anything disappearing for me when I dropped them, so I never had any fear about putting stuff on the ground. Unlike say Skyrim.
(Unfortunately I had a piece of trash that half clipped through the parking lot so now it's just stuck there. Also a curtain with half of it clipped through the basement, so I can't push it through the floor nor can I get it out)