You can adjust the audience of your message by clicking on the second icon below the compose box.
There is four privacy levels on Mastodon:
1. Public: everyone can see your message
2. Unlisted: everyone can see your message but it won't appear in the public timelines
3. Followers-only: only your logged-in followers and the mentioned users will be able to see that
4. Direct: only the mentioned users will see that
Privacy levels are one of the most powerful feature of Mastodon.
Server run by the main developers of the project It is not focused on any particular niche interest - everyone is welcome as long as you follow our code of conduct!