Keyboards listing key-rollover with a specific number of keys can only accept input from that many keys at once.
Keyboards that advertise N-key rollover are able to accept the simultaneous input of all keys on the keyboard.
The old PS/2 connector was capable of full n-key rollover, but early USB keyboards were not.
It is unrelated to the PlayStation 2 gaming console from Sony.
Anti-ghosting
Anti-ghosting is a technology designed to prevent accidental key presses.
It is generally an artificial limit on the number of physically close together keys that can be simultaneously pressed.
It is intended to prevent users from accidentally pressing more keys than intended.
Anti-ghosting is generally a form of 2- or 3-key rollover.
