Favorite Folders are virtual folders that display all messages from a certain folder type (such as Inbox, Outbox, Sent, Unread, Flagged messages, Search Folders or folders based on Tags) from all of your email accounts at once. They can be found in the left pane in the Mail tab, above the folders for your individual accounts.

This feature allows you to check all emails without having to go through each separate account individually. 

You can also pin any of the other folders from your accounts into this section, so the folders you use most often are easily accessible.

These folders can be enabled/disabled in Menu > Settings > General > General via the 'Show Favorite folders' option.

Read more in our blog: Smart folders in eM Client or in our Documentation: About Favorite Folders