A keyboard with n-key rollover 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 some keyboard says 10-key rollover, this means you can press up to 10 keys at once with successful input. So if it says “n” it means any numbers. So n-key rollover is very high-tech features that not every keyboard has.
If you type fast or can type fast? This is the must function you need.