the aim here is to keep a consent voltage available for the led. the input voltage varies between 12v and 15v. The zener by itself cannot handle the load. so it is used to drive the transistor instead.The zener diode and the resistor is what is contained in something like a L7812 voltage regulator.The 200 uf cap is here just to keep a consent load on the output you can change this value. the 100 uf just keeps the input a little more stable.you can add a bigger cap across the input and output depending on your load.
