PWM is a commonly used technique for converting a digital 'signal' to an analogue output.
Try varying the 'width' property of the oscillator at the top of the circuit. Notice at 10% the LED glows dimly, but at 90% it is bright.
The oscillator 'signal' to the base of the NPN transistor allows a greater current to flow from the main power source. The greater the percentage of time the oscillator is 'high' the more current flows to the load.
The capacitor buffers the charge, smoothing the current through the LED load.
