Fediverse addresses look like this:
@ username @ instance
So, for example, the FediTips address is:
@ feditips @ mstdn.social
(but without the spaces)
You can @ people with this address on the Fediverse, and it works just like @ -ing people on Twitter.
A Fediverse address is longer than a Twitter address because the Fediverse is made up of thousands of independent instances, so Fediverse addresses have to include which instances people are on.