Favourites bar
The Favourites bar provides quick access to your favourite or most-used mailboxes or folders such as Inbox, VIPs or Flagged. You can drag messages from the message list to a mailbox in the Favourites bar. To show the Favourites bar, choose View > Show Favourites Bar.
You can also access your favourite mailboxes in the Mail sidebar. To show the sidebar, choose View > Show Sidebar, or press Control-Command-S.