put simply it's the study of media, how it affects audiences, and why it has those effects, but you can go very very broad strokes where you get a lot of overlap with philosophy, or you can go very specific detailed & in depth where you get overlap with social science or more traditional English literature type study.

One of the most famous pieces of media theory definitely leaning towards the philosophical is Roland Barthes's Death of the Author which I imagine you have heard of.