This is a security code that must match the firmware.
While specific instructions vary by model, a common procedure involves these steps:
For some models like the HTC 10 or M9, you can flash directly from a microSD card:
Boot into ; the phone should automatically scan the card and prompt you to install.
Rename the firmware file to a specific naming convention (e.g., 0PJAIMG.zip for M9). Copy the file to the root of your SD card.
The bootloader version; you can only flash a version that is equal to or higher than your current one unless your device is S-OFF.