In the footer of the folder list, the Folder actions icon shows a list of folder-related actions when clicked. Most of the actions in the menu affect the currently selected folder.

This action will compress the folder size by finally deleting messages that have been marked for deletion.
This will delete all messages in the selected folder permanently. Be very careful with this operation as it cannot be undone. For security reasons, this action is only available for the Trash and Junk folders.
Manage folders
This menu item will take you to the settings section when you can manage the mailbox folders. This is where you can create, delete, rename or re-organize your email folders.