The folder list in the email view lists all folder that can be used to sort emails into for temporary or final storage. You manage the folders in the settings section and freely choose the hierarchy of the folder tree.

Special System Folders

Some of the folders have a special meaning and are used by the system to place messages in. These special folders are indicated with an individual icon in the list and usually cannot be deleted or renamed.

This is where new messages arrive.
When composing messages you can save them temporarily as draft. Such messages are stored in this folder an can be picked up for editing here.
Unless configured otherwise, a copy of each message you compose and send to others will be saved to this folder.
When you delete a message it is moved to this folder and not deleted immediately. Depending on the email server’s settings, trashed messages will automatically be removed from this folder after a certain time. You can also empty the trash folder manually with the Empty command from the Folder actions menu.

The assignment of special folders can be changed in Preferences > Special Folders.

Please see Mailbox folders.