Content is the meaning an artist is attempting to communicate to the participant. It is accomplished by how an artist uses form to transform subject matter into that desired meaning. Compare these works that all share the same subject matter, that of Judith beaheading Holofernes.

Consider how different each content is even though the subject is the same.
When subject matter does not exist, as in nonobjective works, then the form becomes that meaning.
Content and subject matter are not the same. Two artists can represent the same subject matter to communicate very different meanings.