For a non-tech answer, it’s basically the “language” used between these websites to make them talk to each other.
If a website uses ActivityPub, it can fetch information (and send information) to other sites that are using ActivityPub in a specific way that’s designed for social media.
Another example of this would be SMTP (simple mail transfer protocol) which is what e-mail uses so that different e-mail servers (outlook, yahoo, gmail, etc) can all talk to each other seamlessly.