Beach erosion is a reduction in the amount of sand a particular beach has.
On a global level, sea level rise causes beach erosion. Beaches get sand from both the ocean and the land. Larger waves move sand from the coastal sand dunes off into the ocean. This raises the seafloor, flattens the overall profile of the beach, and, therefore, causes waves to break further offshore. This, in turn, minimizes the waves' impact on coastal lands.