Subscribed is how an IMAP server decides what folders in your mail account that you can view. You can Unsubscribe and the folder will still exist on the server, but will not show in your folder list. You must be subscribed to a folder to use it in any way.
Checked box means you are subscribed. Un-check to unsubscribe. Return to the Folder page and re-check to resubscribe.
Shaded boxes are folders that are designated as "Special" and subscription status can not be changed. Typically things like Sent and Trash.
