Art is creation. It is a wide concepts that includes Paintings, Drawings, Photography, Literature and Music. Free Art is art that is shared freely. Art that is shared freely is art that helps the process of making more art.
Some art - like paintings and drawings - is difficult to share, because there is mostly only one copy, which is someplace on a museum or at someone's house. Still, also this type of art can be shared by licensing it free of copyright, for example through a Creative Commons license.