N-key rollover, or "NKRO," is a feature of high-end keyboards that detects all keystrokes no matter how many keys are pressed simultaneously. It ensures every keystroke is recorded and prevents "ghosting," in which an extra keystroke may be registered when multiple keys are pressed together.
In a basic keyboard, the electrical connections from the base of the keys to the keyboard output (typically a USB connection) may intersect. If you press multiple keys at the same time, the keyboard may register some keystrokes but not others. Ghosting happens when the data from two or more keystrokes gets combined and registers input from a key you did not press.
Most keyboards have some level of rollover, meaning they support multiple simultaneous keystrokes. For example, an inexpensive keyboard might have 5-key rollover, while another might have 8-key rollover. This type of rollover is also called "x-key rollover," where x equals the number of keys that can be pressed and recorded simultaneously.
N-key rollover means there is no limit to how many keys can be pressed and recorded at once. This feature is typically found in high-end keyboards, such as gaming keyboards used in eSports. Gamers often press several keys at once while playing video games, so it is essential for the keyboard to register all the input accurately.
NOTE: The USB interface initially only supported 6 simultaneous keystrokes plus 4 modifier keys. Modern USB interfaces do not have a simultaneous keystroke limit.