It is the area where the user-created program for programming the on-chip flash memory (user boot program) is stored. When the device is started in user boot mode, the program that is stored in this area will run.
When the product is shipped, the USB boot program is stored in this area. You can write a user boot program to the user boot area in boot mode. However, note that this will erase the USB boot program.
For details, please see Details of Operating Modes in the Operating Modes chapter and User Boot Mode in the Flash Memory chapter of the User's Manual: Hardware.