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.