Fastboot is the bootloader protocol used to flash images on many Android devices.
It works through a host computer’s command-line fastboot tool and a device started in Fastboot mode. Over USB, the tool sends commands that can write binary images to partitions, erase partitions, reboot the device, or report bootloader information. It is part of Android’s SDK Platform Tools, alongside Android Debug Bridge (ADB).
Fastboot is especially associated with bootloader unlocking, custom recovery images, factory images, and custom ROM installation. Unlocking often wipes user data and may require an OEM-specific confirmation, so “fastboot” is not simply a harmless file-transfer mode. It also does not require USB debugging to be enabled, because it operates before the normal Android system has fully started.
A common mix-up is treating ADB and Fastboot as interchangeable. ADB communicates with Android or recovery, while Fastboot communicates with the bootloader. Fastboot is widespread, but not universal: manufacturers can use another protocol, and Samsung devices commonly use the proprietary Odin mode instead.