What is this option for? Enable "Read" and "Notes" in inbox

We recently updated our mail to LN 8. When in my inbox of my mail when I choose Tools > Preferences on the Mail > Basic tab under View and Folder Management I see an option:

Enable “Read” and “Notes” column in inbox.

When I turn it on I seem to see a new Column “Read” which displays different letters. What is the purpose of this column??

The above option appears below: Delete documents in my Trash folder after [ ] hours.

I did a search on this and found NOTHING. When I search for the View and Folders Management section and look at documentation I don’t see this option in the screen prints… What’s up? Are we using a newer template that 8.0.2. for the mail?