The Favorites bar provides quick access to your favorite 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 Favorites bar. To show the Favorites bar, choose View > Show Favorites Bar.
You can also access your favorite mailboxes in the Mail sidebar. To show the sidebar, choose View > Show Mailbox List, or press Shift-Command-M.