Basically with Roam you think of your notes as a graph - when you link to another page, that becomes a relationship in the graph. Technically this is possible with any note-taking app that allows links between pages, but one of the nice things allowed by Roam due to modeling this way is you can see automatically all the pages that link to the current page, as well as instances of the page title that are still unlinked.

This also applies on the block level (you can reference any piece of text anywhere else without copy/paste), though blocks & block references are not currently shown in the default graph view.