Served as "flat files" - filesystem, object store, what have you. No server logic generating content, just passing around of strings and binary data. Files are the representation are the source of truth. Counter to a web app, where the content response is ephemeral and the "source of truth" is scattered across a writeable DB and recombinated (potentially) on every request.

Interesting question though, I (a web dev) just take the term for granted.