A small program that basically “wakes up” the STM chip in your radio. An initial bootloader was installed in the Taranis as shipped.  After flashing the STM chip with OpenTX v2.x, a second bootloader exists on the chip. This bootloader allows communication with the chip through the transmitter keys for special purpose maintenance applications such as USB access  to the settings memory and SD card, flashing new firmware without needing PC drivers, and restoring an EEPROM backup.