This is the movement of sand from the beach to offshore sand bars during the winter, and movement from the sand bars back to the beach in summer. Have a look at Figure 2.3b from Walk Along the Ocean , which shows the difference in profile of a beach from summer to winter, and shows the bars quite well. One of the hands-on activities you will be doing during the beach walk will be measuring the Scripps Beach profile and then comparing it with profiles measured earlier. A Bit on Beach Erosion Beaches are dynamic environments. They change width, height, and other characteristics naturally over time.
Some folks try to stop that, particularly the natural processes which erode beaches.
Folks who want to stop sand from moving often build jetties or groins, which are piles of rock and other material which are perpendicular to the beach. The idea is to stop the longshore drift and currents which move sand. It hardly ever works in the long term. Some coastal residents want to build seawalls to stop the erosion of the cliffs their houses are built on. Of course, they had no business building on the cliffs in the first place, but that, to them anyway, is beside the point.