However, before you can execute ADB and Fastboot commands, there are certain preparations that you must make beforehand. Whether you want to flash a custom recovery or the stock firmware images on your Android device manually, you must execute specific Fastboot commands for the purpose. Using these commands, you can reboot the bootloader, enable or disable OEM unlock, and flash factory images or modify partitions on your device. Fastboot commands work only when your device is booted into the Fastboot or bootloader mode. It works as a communication bridge between your computer and Android device.
Just like ADB ( Android Debug Bridge), Fastboot is a command-line tool as well as a protocol on Android devices.
Flash Update.zip via Fastboot What is Fastboot?
