A move that places "floating rocks" around the enemy team, making each pokemon that switches in (on the opposing team) take damage depending on their resistance/weakness to the rock type. 2x weakness to rock means 25% of max hp as damage on each switch-in. 4x weakness, which many bug types have, mean 50% max-hp as damage upon switching in. And Rock is actually a pretty good offensive type since Flying is so common. The "normal" damage is probably 12,5% if I remember correctly.
In short, it makes every switch you do painful.
...you can, however, remove Stealth Rocks by using Rapid Spin.