 The term key rollover describes the minimum number of simultaneous key presses which can be registered on a particular keyboard. So a keyboard said to have two key rollover can register a minimum of two keys pressed all at once. And a board width N key rollover has no set limit to the number of key presses it can register at once. Some vintage keyboards using old connection standards had N key rollover by default due to the way they reported individual status changes. But you'll often find USB keyboards with a maximum six key rollover due to their communications protocol requiring all pressed key statuses be reported together in every update.