It parked the HDD so the PC could be moved safely
It locked the floppy drive so nobody could copy files
It locked out the keyboard (also booting or opening the case)
It switched the display between color and monochrome
Choose wisely! The correct answer, the explanation, and an intriguing story await.
Correct Answer: It locked out the keyboard (also booting or opening the case)

And now, for some context

An actual key: The 1980s answer to pressing "Win+L"

If you ever cracked open a beige 386 or 486 tower, you might remember a keyhole on the front panel, usually parked near the Turbo and Reset buttons. IBM called it the keylock. With the key turned to "locked," the PC stopped listening to the keyboard. It was a physical password.

The reason it existed is explained by the software of the time. Early IBM-compatible PCs ran MS-DOS, a single-user system with no accounts and no passwords. Anyone who sat down at your desk could browse your files (or delete them) with a keypress.

IBM had already used keylocks on its mainframes and servers, and in 1984 it brought the idea to the IBM PC AT (model 5170), an expensive 286-based machine that cost thousands of dollars (over $12,500 in today's money). The AT shipped with a tubular pin-tumbler lock, similar to the "Ace" locks found on vending machines, pinball tables, and alarm panels. PC Magazine described it as a way for executives to sleep at night.

On that IBM, the lock did several things: it blocked the keyboard at the controller level, so software couldn't work around it. If the machine was powered on while locked, the BIOS self-test halted, and a metal piece inside the case moved into place to stop the cover from being removed. IBM also sold a separate "Keylock Option" for the original PC and XT that went further and blocked the machine from powering on at all.

Compatibility was the selling point of PC clones, so clone makers copied the feature, but not faithfully. Most only disabled the keyboard, and the case-locking part was reserved for higher-end systems. Implementations varied by maker, with some locks cutting off the power supply or hard drive instead. In some cases, budget builders skipped the wiring altogether, so the lock was purely decorative.

The keylock is a passing memory these days. It faded in the early-to-mid '90s, along with the Turbo button, as passwords arrived in software. Windows NT, NetWare, and Linux had real user accounts, Windows 95 had profiles (though not real security), and BIOS passwords became a common feature.

The physical lock idea lives on in cable locks like Kensington's, in the lockable front panels of servers, and in the keyboard shortcut you now use to lock your screen.