You lift a heavy medicine ball (typically one that doesn't bounce or you'll knock yourself out) and you lift it and slam it down repeatedly.

Yeah, it’s a weighted medicine ball that is partly deflated. You pick it up, raise it fully overhead and slam it down at the same time squatting and catching the ball as it does a tiny bounce, then repeat. Technique is everything, but it’s a great cardio movement, especially when combined with 2-3 other moves.