Hi Abiram,
Suppose you have a 12V DC motor, and you are supplying 12V DC consistantly for the motro. In that case the motor will run at its maximum rated speed, say 3000rpm.
If you apply this 12V at every one millisecond with a delay of 1ms where there is no supply, the motor will run at half the speed, 1500rpm. Here the ON TIME of the supply is 1ms and OFF TIME is also 1ms. If you increase OFF TIME to 1.5ms and reduce the ON TIME to 0.5ms the speed of the motor will again reduce to 750rpm.
Hence if voltage is applied in pulses the speed depends on the ON TIME and OFF TIME. This is the concept of PWM or PULSE WIDTH MODULATION.
PWM is a wave with constant pulse PERIOD which means (ON TIME + OFF TIME) is a constant. To increase the speed simply increase the ON TIME and decrease the OFF time. To decrease the speed, reduce the ON time and increase the OFF time.