Self explanatory, a pool filled with plastic coloured balls where kids gather inside to wade around in as they pelt other kids with balls. More deviously used is for pedophiles
to hide in futilely to molest young children or to fap in the pit. Needless to say, with a combination of piss, bacteria and sperm.
Damnit, this was the third adult we had to escort out of the place today for hiding in the ball pit.