The bootloader mode is the hidden menu of Android system. Usually, there are two methods of entering the bootloader: by using the secret combination of keys or by using the commands on PC. Bootloader is a set of codes which is executed every time an operating system begins to run. It instructs the system kernel to boot normally. The bootloader is usually locked by the manufacturers to prevent the users from changing the version of the operating system which is specially designed for the particular device.
You may use the Bootloader Mode is the following situation