N-key rollover
Also called NKRO · 6KRO · key rollover
Key rollover is the number of keys a keyboard can register at the same time. A keyboard with N-key rollover registers every key you hold at once while a 6KRO keyboard stops at six.
The keyboard test reports your peak rollover: the most keys it saw held down together. USB keyboards running in boot mode are limited to six regular keys plus modifiers, so a lower number than the box promises can be a mode setting rather than a fault.
In practice
Rollover only matters when you hold several keys together. Typing rarely needs more than two or three at once, so most people never hit the limit. Gamers do: running diagonally while crouching and reloading can mean five keys held together. On a keyboard with a low rollover limit the extra keys are simply dropped and the game never hears them.
How to check
Hold down as many keys as you can in the keyboard test and watch the peak count. If it stops at six, the keyboard is probably running in its USB boot mode or is a six key design. Try the keyboard’s NKRO setting or a different USB port. Then hold the exact combination you use in your game, since that is the one that has to work.