@cwebber let’s try this way:
ActivityPub is the name of a technical protocol. Like HTTP is for websites.
Fediverse is a short word for Federated Universe. Which means software using ActivityPub can communicate which each other and form a kind of universe. Like Web browsers can all access the same Web contents/sites.
There are several Fediverse compatible software focusing on various subjects (messages, photos, books, etc). You may access their content from any of those.