This is a folder shown at top of Account list.
This folder creates a special Message list with messages from all selected accounts. So this folder shows messages from various selectable folders.
By default, each Account's INBOX folder is shown in the Combined folder. However, you may configure to show arbitrary folders in the combined folder. To select folders, open context menu on combined folder in the Account list, and click Folders button. Then select folders that should be shown.
Note: if you click on account in this dialog, then account's INBOX folder will be added or removed from selected folders.
When you view list of messages of combined folder, you have another possibility to change folders that will be shown in the list. Selection made here is not preserved, and it will affect only current session of message list.
If desired, you may hide Combined folder from Account list in Configuration.