A "shell beach" is a sea beach that routinely has an unusually large accumulation of seashells washed up on it. Seashells are most often the dead empty shells of marine mollusks, but may also include tests or shells of other kinds of marine animals.
The majority of beaches in the world are primarily composed of rock particles such as sand, grit, gravel, pebbles, etc, but in rare cases (including Shell Beach in Western Australia) a beach can be composed entirely of seashells, both broken and whole valves.
One area in the USA that is famous for its shell beaches is Sanibel Island on the Gulf Coast of Florida. In South Africa, the beaches of Jeffreys Bay are famous for shells, as is the Sulu Archipelago in the Philippines.