An example is email. You can send an email from your Gmail account to a Protonmail account, even though they’re on different networks

So with federation on Lemmy, people from Lemmygrad.ml can post on Lemmy.ml and vice-versa, even though they’re different servers on different networks. You can also spin up your own Lemmy server and federate with them

There’s work in place to get it to work with ActivityPub, so your Lemmygrad posts could be liked by users from Mastodon servers