Erosion is when water hits across a surface to wear it down little bits at a time, a common example is when water hits a cliff face. In the water (sea) there are materials that have come away from the surface. When the water hits the cliff face, so do the materials. This eventually creates an overhang. Over time the overhang collapses leaving more material in the water and the process repeats itself.
That's the best i can do, hope it helps!