Memes

A common way I've heard it explained is to think of it like email. Say someone creates a gmail account and their friend creates a hotmail account. The gmail user doesn't need to create a hotmail account to send them a message, they just log into their gmail account and send their message to friend@hotmail and the magic of email takes care of the rest.

The fediverse works similarly. You create an account on one "instance" and can interact with pretty much every other instance. There's some nuance to it since it's more complicated than email, but that's the gist of it.