You can set playlist visibility to public or private.
Public means anyone can see your playlist.
Private means the playlist is only visible to the owner and collaborators of the playlist.
You can set a playlist to be locked or unlocked.
An unlocked playlist can be opened by anyone
A locked playlist can only be opened by the owner and collaborators of the playlist. Everyone else will see a dimmed playlist thumbnail with a padlock icon and will not have access.
Why would I want to lock my playlist?
A teacher for example, may choose to give students a portion of their curriculum; however they may choose to keep the rest of the course hidden until the students complete the first part of the course.