The most common version is the UEFI - ("Unified Extensible Firmware Interface") - a separate operating system, often running on its own chip (though sometimes running in a notionally "secure" region of your computer's main processors) that is charged with observing and securing your computer's boot process.
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UEFI poses lots of dangers to users; it can (and is) used by manufacturers to block third-party operating systems, which allows them to lock you into using their own products, including their app stores, letting them restrict your choices and pick your pocket.
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