What is Hboot/Bootloader?

Hboot, or we may call it Bootloader, is like BOIS to windows. It is the first thing that runs when you boot up your Android device. It packages the instructions to boot operating system kernel and most of them are specifically designed to run their own debugging or modification environment. Basically, Android device is like a hard drive that has several partitions, which holds the Android system file in one and all the app data in another. And for all these partitions, Bootloader (or Hboot) serves as a security checkpoint that is responsible for checking and initializing the hardware and starting software. It can also be used for flashing official software releases.

Lock & Unlock

Most bootloaders are locked, as well as encrypted, including the developer-friendly Nexus devices. Manufacturers usually lock the bootloader to make sure you stick to their Android OS version specifically designed for the device. With a locked bootloader, it is virtually impossible to flash a custom ROM and other forced attempts, which would void warranty as well as end up in bricks.

Nexus devices and a couple tablets can be easily unlocked with a single command. Others, however, couldn't be unlocked with just "fastboot oem unlock" command. HTC, as we know, has provided an official tool that will unlock the bootloaders on its devices. Other manufacturers have done or try to do the same thing.

Note: Unlocking bootloader voids your warranty and also completely wipes out the internal memory including contacts, messages, applications, etc.