This occurs when waves approach the beach at an angle.
The swash brings material up onto the beach, then the backwash brings it back down to the sea in a straight line. The swash, again moving in at an angle will move the material further across the beach.
This process continues and the load is moved along in a zig zag pattern.
A beach is made of sand and shingle (small stones) which are deposited by the waves.
The swash carries material up onto the beach. Some of this material, especially the heavier material, stays on the beach because the weak backwash cannot carry it back to the sea. The backwash carries only lighter sand particles back down towards the beach.
For this reason we say that beach material is sorted, with the heavier materials furthest from the water, and the lighter sand close to the water.
During storms, when the swash is very powerful, it can force heavy stones high up onto the beach where they remain to form a storm beach.