A keyboard with n-key rollover, or abbreviated as NKRO, has the ability to scan each button press individually, as opposed to having the PC do it. As a result, every pressed button, or key, is noted, even if you’re pressing a bunch of keys simultaneously.
Sometimes you'll see the "n" in n-key rollover replaced with a number. That number tells you how many keys you can press simultaneously with the keyboard being aware. For example, if your keyboard has 6-key rollover, you can press six keys at once with successful input. N-key rollover is particularly relevant/helpful for gaming keyboards.
Not all keyboards come with n-key rollover, as implementing the feature comes with certain cost and design challenges.
This article is part of the Tom's Hardware Glossary.