What is meant by Unlocking Boot Loader? When a computer starts up you can press a button like F2/F8/ESC key and enter to the BIOS menu (Boot menu). This is also a some kind of boot loader. For another example, if you have installed multiple operating systems in a single computer then a menu will be displayed at startup asking which OS to boot. This is also called a boot loader. In Android phones there is a similar boot menu. But this menu has been locked by default.