Think of the Fediverse as the world and each instance as a different country. Your account on the Fediverse is like your passport, so while you only have one passport belonging to your home country (Your home instance), you can use that passport to travel to different countries around the world.
Lemmy, Mastodon and PixelFed are like the different cultures of the world, while each place has its own way of displaying content, you can travel between them and see them all the same.
Here’s my attempt to explain the fediverse:
Imagine Reddit.com was actually RedditWC.com and RedditEC.com for East and West coast. West coast would have a Seattle community and East coast would have a New York City community. You might be more interested in Seattle so you sign up at RedditWC.com.
Either way, the two sites are constantly sharing all the content created on each so they're a perfect copy of each other so you’ll still be able to participate in the New York City community. This scales to allow a California instance and a Texas instance, etc., each with their own interests and all posts and comments are being synced.
When you load up RedditWC.com, you're seeing everything created on RedditWC.com as well as up to date copies of all content everywhere else you can interact with so it still feels like a complete Reddit.com.
The crux of all of this is each instance has less traffic so the load is spread out, and if instances go offline, other instances can pick up the slack.
