noun
art supplies; art materials
Refers to physical materials used for drawing or painting, such as paints, brushes, canvas, etc.
画材屋で新しい絵の具を買った。
I bought new paints at the art supply store.
この画材は初心者にも使いやすい。
These art materials are easy to use even for beginners.
noun
subject matter of a painting; motif
Refers to the theme or subject depicted in a painting, not the physical materials. Less common than the 'art supplies' sense.
彼の作品の画材は主に風景だ。
The subject matter of his works is mainly landscapes.
Compound of 画 (picture, painting) and 材 (material, subject). The 'art supplies' sense is the most common modern usage.