Marking a mailing or list as a favorite allows this mailing or list to appear at the top of your Lists or Mailings tab. This overrides the sorting preference you have selected and keeps things marked as a favorite at the top. Having a mailing or list at your fingertips makes it quicker to send the same mailing or send to the same lists that you use more frequently.
To mark a mailing or list as a favorite, simply go to your Mailings or Lists tab and use the Star icon (★) next to the mailing or list name. This will mark that item as a favorite. To deselect an item as a favorite simply click the star icon again to deselect it. Favorites are limited to 4 for each tab.