https://www.kingoapp.com/root-tutorials/how-to-unlock-bootloader-android-device.htm (From PHX Browser)
What Is a Bootloader?
An Android device consists of several pieces of software, which include the bootloader, radio, recovery, and system. The bootloader is the first piece of software that is run when your device turns on. It decides whether to load the recovery, or load Android and the radio. It packages the instructions to boot operating system kernal and most of them are specifically designed to run their own debugging or modification environment.