This simply means the type of media text or media platform that we are studying. For example, a magazine is a different media form to a TV programme or a website.
The media language we use to analyse a media text will change with different media forms. For example if we were analysing a film, we would talk about camera movement, editing, sound, location, props and mise-en-scene, where as if we were analysing a newspaper, we would talk about headlines, mastheads, captions, bylines and cropping.