In art, "medium" refers to the materials and techniques an artist uses to create a work. It's essentially what the artwork is made of. For example:
Paint (oil, acrylic, watercolor)
Clay (for sculpture)
Pencil or charcoal (for drawing)
Film (for photography or video)
Digital tools (for computer art)
So, when someone asks about the "medium" of a piece, they're asking what the artist used to make it.