Who can see your posts
The four audiences, what each one means, and what happens when you change your mind.
Every post carries its own audience, set from the globe in the composer. Your default is in Settings → Profile; the composer starts there and you can change it on any post.
Everyone Copy a link to this section
Public. Visible to anybody, signed in or not, and the only kind that appears in search engines, the sitemap or a hashtag page seen by a stranger.
Followers Copy a link to this section
Only people who follow you. If somebody unfollows you, the post stops being visible to them.
Mentioned only Copy a link to this section
Only the people named with an @ in the post itself. Useful for a note to two or three people that does not need a message thread.
Only me Copy a link to this section
Nobody but you. It sits on your profile where only you can see it.
Changing the audience later Copy a link to this section
Open the ⋯ menu on your own post and change it there. Narrowing it — from Everyone to Followers, say — takes the post away from anybody outside the new audience immediately.
What it cannot do is un-read what somebody already read, or delete a screenshot. Narrowing an audience is a change to who can open it from now on, and it is worth being honest with yourself about that before posting rather than after.
Two things the audience does not control Copy a link to this section
Replies to your post carry their own audience, chosen by whoever wrote them. A public reply to your followers-only post is public.
A post that has expired is visible to nobody at all, including you — expiry is checked before the audience is.