The folder at the top is the RexxMail main folder. It comes pre-filled with a number of objects, from left to right:
The main folder also contains the other RexxMail folder objects shown here: the 'Addresses' folder, the 'In' and 'Out' folders, and the 'In Archive' and 'Out Archive' folders. In addition, the main folder contains a couple of folder objects not shown here: the 'Configuration' folder and the 'Temp' folder.
The main folder is a work area folder, so closing it will also close any open objects it contains.