Abrasion is the wearing away of rock in a rubbing action, something like natural sandpaper. It happens when moving water or ice carrying small rock particles flows over bare rocks.

You can see what happens when two pieces of rock are rubbed together by trying this test. The large pieces are slightly worn away and fine powder is produced.

Abrasion, or rubbing, happens when any material knocks together.

The effect is much the same as using sandpaper. Rivers carry fine rock particles (sediment) that abrade the banks and bed of the river channel. At the same time the material bumps together, making it finer and more rounded. This is called attrition.