In art, a "medium" is like the tool or material an artist uses to create their artwork. It's what they use to bring their ideas to life. Think of it like ingredients in a recipe. Just as you might use eggs, flour, and sugar to bake a cake, an artist might use paint, clay, or pencils to make their art. So, when someone says "medium" in art, they're talking about the stuff used to make the art piece.
* Artists are always innovating, so there's no limit to the materials one could potentially use in their creations. The only boundaries are safety and respect for the environment and others.