The Favorites Bar appears just under the toolbar, and it provides a way for you to easily slide the Accounts / Folders pane open or closed, in addition to quick access to accounts and folders. The Favorites Bar is particularly useful if you wish to keep the Accounts/Folders pane closed for power email triaging!
To show or hide the Favorites Bar, navigate to View menu > Show > Favorites Bar.
To add a folder to the Favorites Bar, select the folder in the folder pane and drag-and-drop it into the Favorites bar. Once added, you can then rearrange folders as desired.
You can even add Account Groups to the Favorites Bar, which will give you fast access to the Inboxes for each account within the group.
To remove a folder from the Favorites Bar, right click on the folder and select Remove From Favorites from the contextual menu.
To file a message, simply drag-and-drop the message to the Favorite folder.