A Human Hamster Ball is similar to an ordinary hamster ball

Hamster ball

Hamster balls are hollow spheres made of clear plastic into which hamsters, gerbils, degus and other small rodent pets are placed, allowing them to run around outside their cages without the risk of running away, getting lost under furniture or in walls...

but much larger, so that a person can get inside. The game is played to test your ability to keep balance in the ball while running. Most of the time, this game is played in a pool of water. To get in you have to hurry and jump in before it deflates and water gets in.