It's decentralized twitter basically... There's more nuance but for now just consider it twitter for this examples sake.

So imagine everyone can host their own twitter server, each server connects to each other... Or inversely not connect to certain servers if they want.

Now instead of a single server being a catchall for everyone, you can have nuanced servers.. since you have both a local timeline and a federated "all servers" timeline as well as the timeline of people and hashtags you follow.

Let's focus on the local and federated timelines.

Let's say you're on a mastodon instance that's all about pokemon Minecraft, people on this server will probably share that same like.. so when you look at the local timeline you're seeing people only on your pokemon Minecraft server.

When you look at the federated timeline you're seeing EVERY other servers feed.

So instead of seeing mastodon as a replacement for twitter, ask yourself, what kind of community do you want to be a part of.... Then find that using the mastodon community finder, go sign up and post a introduction and you'll find new people on the local timeline.

Think of it kind of like forums, but twitter.