Firmware password

A firmware password, also known as EFI lock and formerly known as Open Firmware Password Protection, is a low-level password that can be stored with the firmware of Macs that support Open Firmware or Extensible Firmware Interface (EFI) since Mac OS X 10.1.

Functionality
A firmware password can be enabled or disabled from the macOS Recovery partition and prevents the designated startup volume of the Mac from being changed. The Lost Mode feature of Find My Mac also allows a firmware password to be remotely set. If the password has been forgotten and needs to be reset, the device and proof of ownership will need to be brought to an authorized service provider or the Genius Bar at an Apple retail store.

Articles

 * Bypassing EFI Lock on Your Mac at Best Reviews (2017-12-21)
 * How and Why to Use a Firmware Password by Jacob Penderworth at Envato Tuts+ (2014-06-17)