Basically, how many keys your keyboard is capable of handling simulatenously (yes, there is often a limit, mine has 3, some can have even 2). There will be no more than this many keys for you to press. Select 2 and you'll be safe!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.