In the simplest terms, a bootloader is a piece of software that runs every time your phone starts up. It tells the phone what programs to load in order to make your phone run.
The bootloader starts up the Android operating system when you turn on the phone. This is a pretty important job, so it’s very important that nothing goes wrong with it. That’s why phones keep their bootloaders stored in special stable memory.
Alternatively, the bootloader can start up recovery mode. When a phone is in recovery, it can execute large pieces of code that totally rewrite the Android operating system.
The bootloader is important because it loads both of these pieces of software. Without a working bootloader, your phone is a useless brick.
Why unlocked bootloaders are important?
Some bootloaders, however, contain an extra layer of security that checks to make sure that it only loads an operating system that passes its approval process.
The exact process of checking for unauthorized software varies from phone to phone, but the effects are the same no matter what. You can’t run any operating system the manufacturer did not approve.
A locked bootloader means you can’t install your own OS. For most people, that’s fine. However, if you want to put the latest version of CyanogenMod or any other custom ROM on your phone, a locked bootloader means you can’t.
Locking the bootloader is a very serious practice with some serious implications. It can present a challenge to Android developers and enthusiasts who wish to install custom ROMs or change the phone.
Installing a locked bootloader is becoming less common, although it remains far too prevalent. It can vary from phone to phone and even from carrier to carrier on the same device.
Google seems to understand and respect the users’ wishes. For example The Samsung Galaxy S IV with an unlocked bootloader and stock Android was a step in the right direction.
Verizon is especially bad about releasing phones with locked bootloaders. For whatever reason, it feels the need to lock down its phones and prevent users from taking advantage of everything it can do for them.
ChimeraTool can unlock, relock and lock the bootloader to the original factory state for the supported models you can see below. It is a one click operation, where you can select the lock level you would use.
By this way you can unlock your bootloader and use any custom ROM, 3th party firmwares, but if you would revert back to the factory state due to warranty purposes for example, you can Lock back the bootloader.