These clay pebbles are nothing but the tiny clay balls undergone high-temperature heat. When fired in a kiln these tiny pebbles expand and become porous from within. Some people also call them Hydroponic Clay Balls, Hydroton, clay pebbles, or light expanded clay aggregate (LECA).
hydrotons are great water absorbers, it collects the excess water and stores it for later use. It prevents roots from being damaged due to excess water. Generally, clay pebbles are used as a base layer or alongside to help plants get the proper amount of water and air through it.