Pulse Width Modulation, is a DAC (Digital Analog Convertor) using only numerical components (outputting only 0v or 5v values)<br />
Using a entry clock, you output a signal 0v or 5v in order to result an average fix value between 0v and 5v, for sample 2.3v. Changing like it the speed of a small electric motor as you want but using at input only two values : 0v or 5v.
Using a quicker clock, you can generate analog signal, like sound music.