an ELI5:

When you set up a lemmy instance, it has no idea other instances exist. It's like throwing you into a web browser with no search engine. You don't immediately see every single website, you have someone tell you about a cool website you found, and then you type it into the address bar, and save it.

It's kind of the same thing with Lemmy instances and communities. Once a user types this syntax into the search prompt:

[!community@instance.com](/c/community@instance.com)

It will try and contact instance.com for that community. If it exists, the user can subscribe and the instances will now receive and send new posts to each other.