@Charmer Mastodon is like a email service, you can set up as many servers. If you disable federating people won't be able to see you who set up on a website that's not SPECIFICALLY mastodon.social (for example kickass.zone) and your "toots"/tweets will only display here AFAIK.

@Charmer in case no one else more qualified chimes in: my understanding is that it means members of other "instances" of the GNU Social network (Mastodon.social is one of these instances) can post by default to any other "instance". each instance being a sub-network of the greater federation, hence the terminology

@Charmer basically, this site you're on (mastodon.social) is one server. your full username is actually @Charmer. sometimes, depending on who's connected to who (it's not entirely clear to me yet), this server connects to other similar servers, and publicly listed posts get copied back and forth between them, and displayed that way. this happens directly if you try to engage (eg reply or follow) someone on another server as well.