@alainamartin Well, in simple terms, what we have here is basically like email. With email, you can be on outlook, Gmail, protonmail, etc. But still are able to talk to other emails no matter which platform you choose.
However, it IS way more advanced than email in the fact that there's MUCH more here that's "federated" than just text/media posts like this.
If you read the "Software using ActivityPub" section of the ActivityPub Wikipedia page, that should give you some ideas about what is possible thus far.
https://en.m.wikipedia.org/wiki/ActivityPub
Note: the list is more than likely incomplete and also contains some dead software (hasn't been updated and probably won't be) but it's a good place to start.
https://en.m.wikipedia.org/wiki/ActivityPub
If you have any specific questions, feel free to ask! I'm very passionate about this place, as you'll quickly learn.😁