The Android Recovery System is a hidden mode built in most Android device. You can enter the recovery system by using the secret combination of keys like Volume Up + Power key, Volume Down + Power and others. This mode helps you repair your installations and install an official operating system's updates.
In most Android devices in the recovery mode you can: fastboot the device, find more info about it, wipe personal data, reset cache partition, update system, check root integrity and accomplish other advanced operation. You may also customize it to do so much more. The set of options depend of brand, model and Android version that you are using.
You can find more video tutorials here: HardReset.Info YouTube Channel