Before Apple refreshed security on Intel Macs with the T2 Security Chip and FileVault, the company offered a different way to deter certain kinds of physical attacks. Intel Mac owners can set a firmware password, which prevents a volume other than the one set to start up the Mac from being used on restart or from a cold start.
Apple did away with the problem entirely with Apple silicon by adding low-level security policies and other system drives that prevent an M-series Mac from starting up with an arbitrary volume. (This also made it difficult to have a bootable backup in case of a drive failure, but SSDs—the only built-in option for M-series Macs—are far more reliable than hard drives.)
If you have or are planning to buy an Intel Mac and want to know how to remove the firmware password, it’s an easy process.
Read more at Macworld.com