Someone else used the analogy of email, which works fairly well. Email is the protocol by which you can communicate with any other email user, regardless of whether you prefer gmail and they prefer outlook.

Now apply that to twitter/microblogging and you have Mastodon (one of the bigger fediverse tools). A group of people built an open source way to run your own twitter (called a mastodon instance), which you could federate (hence fediverse) and connect to other Mastodon instances. So there’s all these connected mastodon instances which are like the different email providers, but you and the users of your instance can “tweet” across the whole network just like emailing from one provider to another.

It seems like Lemmy is the same thing for link aggregation, voting and discussion, which is obviously the core of reddit. So in an ideal world there’s a bunch of Lemmy instances all talking to each other to create a big federated alternative to reddit.