How it works
A setting is a switch that controls how an app works. Some settings choose who sees your posts, and others choose who can message you.
Think of rings around you, like ripples in a pond: only me, then family, then friends, then everyone. People who see a post can save it or share it on, so a big ring is hard to take back.
A default is what a setting is set to before you change it, and it is not always the most private choice. So check before you post, not after.
Friends only is a small ring, but only as safe as its list. Look again every few months, with a grown-up.