People new to Mastodon, are you confused by all the different sites and servers you can sign up on?
Don't be, it's really easy!
You know email, right?
With email, you go to an email provider, sign up and they give you an address. Then, when someone wants to email you, you tell them your address. It doesn't matter if they use a different provider, because all the providers talk to each other.
Well Mastodon is the same, but with social media!
You can sign up on any Mastodon server and you'll get an address with that server's name at the end, something like @ username @ server
For example, I signed up on mstdn.social and my address is @ feditips @ mstdn.social
People who signed up on other Mastodon servers can interact with you by @-ing this full address. You can see people's addresses on their profile page (see the attached image for an example).
Mastodon servers are also known as "instances". It doesn't matter which word you use, they are the same thing.