When Num Lock is on, the Numeric Keypad responds with numbers, as an adding machine. When Num lock is off, the Numeric Keypad responds as navigation keys using the alternate markings on the keys.
Num Lock is a key on the numeric keypad of most keyboards, used to switch that pad between number entry and arrow keys. It is a toggle key, like Caps Lock and Scroll Lock. Its state is usually represented by a light on the keyboard.