People tend to explain Lemmy using an email analogy.

Lots of people can use email by signing up with different email hosts. You can send and receive email from anyone as long as both parties know each other’s email address. You can look at all of the email you receive by just logging into your account. When one person sends an email, the various email hosts route the email until it finally ends up in the receivers inbox.

Lemmy is like that, except all of the Lemmy servers (instances) don’t communicate with an email protocol. They communicate with a special protocol that lets them talk about link aggregation stuff (posts submitted by users, comments, upvotes/downvotes , available communities, etc).

It doesn’t matter which Lemmy instance you register on so long as all of the other Lemmy instances you’re interested in are connected and communicating (aka federated) with the Lemmy instance you’re registered with.

Hope that helps!