Damping
A drag that drains a little energy each step, so bounces and waves settle down.
See it in Liquid glass. Drop a few ripples, then raise Damping and watch them fade faster.
In plain words
In the real world, motion slowly dies away. A swing stops, a bell goes quiet and ripples on a pond fade. Damping copies this in a simulation by removing a little energy every step. With no damping, waves keep bouncing for ever and can grow wild. With too much, nothing moves for long.