Light is used in art to form shadows and such. Normally there is a light source, or is is a pretend light source so that it can be used. The exact definition is, "the illumination coming from a light source." It is used in art to show depth and value, as well as making an image look realistic.
Hope this helps. I felt like this needed a better answer that answered the whole question. Also one able to be used for ed2020. Have a nice day.