Basically, you can be on (almost) any server, with its own policies, and communicate with/follow anyone on (almost) any server, just by putting their server name after their address (if you're on mastodon.social, you could just use @bhtooefr, but if you were on, say, witches.town, you'd have to use @
[email protected] (but if you were following me or conversing with me, it'd likely autocomplete that)), similar to emailing.
The reason I say almost any server is, server admins can choose to block other servers if they're receiving a lot of objectionable content from those servers. (Or, they can block specific users on other servers, too.)
The idea is that you get on a server that matches your interests (not actually necessary, but it makes the server's local timeline more relevant) and desired rules, and then you're in the network.