Bootloader
Low-level software that runs right when a device is powered on. It is responsible for loading the the operating system (OS) software, which in turn manages general operation as well as all other software on the device.
See: OS
Some phones come with a locked bootloader, which means it will only load the OS installed by the manufacturer (and manufacturer-approved updates.)
Other phones come with an unlocked bootloader, or the manufacturer officially offers a way for consumers to unlock the bootloader. An unlocked bootloader allows the user to install a different OS, with enhanced and/or customized functionality, such as those created by enthusiast communities.
Using an unlocked bootloader to install a different OS usually voids all or part of the manufacturer warranty.
