It basically turns the motor on and off really fast. Think about it, if you turn a light on and off really really really fast (faster than you could do), it would appear dimmer than if it was just on. The more you have it on the brighter it would be, and the less you have it on the dimmer it would be. The same thing with a motor. The "duty cycle" determines how long it's ON and how long it's OFF. A 50 50 duty cycle is on half of the time off half of the time. A higher duty cycle would make the motor go faster, and a lower duty cycle would make it go slower.
This sounds like a homework question? If it is, you should really be doing it yourself.