ELI5

If your house is your local computer, the place that you're accessing from (kbin.social) is like the post office in your town. You know how email addresses follow the format <user>@<hostname>? The same kind of thing is taking place here - kbin.social is it's own place, you log in there, and it has a local ability to moderate your posts and what you see.

The difference with ActivityPub is that instead of only seeing the mail that was delivered to you and knowing nothing about what other people's email looks like, you see the whole "town square" of kbin, and events in cities that it's connected to as well. Depending on what software is being run, that can be presented in different ways, because ActivityPub is flexible and focuses mostly on the idea of "sharing and subscribing to events" - it doesn't say what the interface has to look like.

kbin makes it look like reddit, but things happening on kbin are also represented on, for example, Mastodon. When I look at kbin's "Microblog" tab it shows a lot of accounts that I am familiar with from Mastodon. They don't live here, they aren't posting here, but they're "federated" here and you can interact with them.