NXP programs every NHS31xx IC with firmware which acts as a second stage bootloader. It provides the functionality to program the final firmware in the IC via the NFC interface, allowing late-programming outside a production environment.
This APP implements the communication protocol to interact with this initial firmware on the NHS31xx ICs.
You can choose from the demo list NXP provides, or you can make your own demo's available in the app. The selected firmware image will be sent over the NFC interface to the NHS31xx IC. When download is finished, the second stage bootloader is no longer available: the IC is reset and the new application executed.