Mastodon is, basically, Twitter with multiple servers (so, if one server fails or goes rogue, it won’t fail for everyone else), a higher character limit per toot, actual rules against hate that are enforced by community moderators, no ads, no algorithmic suggestions, and a lot less bots trying to trick people into voting against their own best interests.
If you can get over the one extra step involved in creating an account, it’s a better Twitter than Twitter. And it’s been growing quickly in the last few months.