If you're coming from Twitter, you might not recognize the "post privacy" option.
Basically, each post you send on Mastodon can have 1 of 4 privacy settings: Public, Unlisted, Followers-only, or Direct.
Basically, Public goes far and wide, Followers-only is nice for more personal stuff (and can't be boosted -- retweeted), and Direct means it's like a Direct Message to any and all users you mention (tag) in your post.
Dear WordPress folx, If you're considering leaving Twitter for Mastodon, you should know that you can set your site up like a pseudo-Mastodon instance using @pfefferle's excellent plugins: ActivityPub and NodeInfo.
cipm, PhD, LLM / Wrote my Ph.D. thesis about Vendor Lockin.