The word "substrate" is a combination of the two latin word parts "sub" for "underneath" and "sternere" for "to put on the ground, to lay something under". So, the meaning of the term "substrate" becomes apparantly "what has been put underneath" or "the underlay". Underlay for a successful plant cultivation under artifiial light is not only good electric equipment, but also a good substrate as a medium. There are substrates of various kinds: while soil is still one of the most popular media, rockwool, coconut fibers and clay pellets have earned their place in the growers hearts in the wide field of hydroponics.
Expanded clay pellets are used as a substrate in hydroculture / hydroponics, or as a soil additive. They can also be spread as a top layer over soil around plants to prevent the growth of unwanted weeds.