Pulse-width modulation (PWM), is a technique utilized in robotics for controlling motors and servos. Through the use of internal counters, the microcontroller modulates the duty cycle of a square wave to control the amount of power delivered to a device. The Duty Cycle referes to the porportion of time the square wave is 'on' as compared to the repeating signal period. The higher the duty cycle the higher the power carried in the signal. Duty cycle is expressed as a percentage of time the signal is 'on', with 100% being consistently on.
Once you feel comfortable that you understand the concepts behind a duty cycle signal, return to Reference Manual: Timer Information portion of the Drive A DC Motor Tutorial