Just had someone asking what instances are.
It's really, really simple:
The instance you are on is the site you signed up on.
Fediverse instances are connected, so that people on different instances can interact with each other. (It's kind of like phone network providers, even people using different providers can still call each other.)
Different instances have different owners, different rules etc. If you disagree with how an instance is run, you can move your account (including your follows & followers) to a different instance.
You can see which instance someone belongs to by going to their profile page and looking at the Fediverse address.
For example, FediTips is on the mstdn.social instance.
There's a really good animation from a couple of years ago which explains all this:
@feditips more easier expla nation:
Instance is like email provider. You can have your account on any provider (gmail, yahoo, proton), yet you can communicate with all others.
I usually explain the fediverse by saying "the fediverse is a network of different websites, kinda like email". And I think most people gets it.