I noticed that my two recent posts have a key symbol beside the title, and hovering the mouse says the post is protected.

I now see that there is a Password Protection tab at the bottom of the post editor. Clicking that reveals a place to enter a password to "lock" the discussion. Is that the same as "protected"? I notice that there are asterisks in the password field, but I never entered any password. Where did that come from? (I removed it from this post to see if it makes a difference).