If an internet post is public, anyone can see it. To control who sees your posts, you need to use a non-public visibility setting.

On Mastodon, the non-public visibility settings are:

🔒 "Followers only" - Only your followers can see this post. If you use this, it's important to also switch on follow requests. Otherwise anyone can follow you to instantly see your followers-only posts.

@ "Mentioned only" - Only people you @ within the post will see it.

More info: https://fedi.tips/how-to-use-mastodon-and-the-fediverse-basic-tips/#WhoCanSeeMyPosts