The two sources represent a badly regulated power supply, having some voltage ripples. In this example we use Zener diode to stabilise the voltage for the rest of the circuit (the load: bulbs)
Resitor = limits current through zener diode and when there is no load, the whole circuit. Needs wattage high enough not to burn out and resistance to keep the diode within it's operating limits (max A)
Zener = reverse biased, limits/stabilizes/clips output voltage that is taken across (see the Voltmeter). It transforms any extra voltage that is above the zener voltage into current flowing through the diode, keeping the output voltage stable. See the graph to visualise diode's effort.
If the input voltage is below the zener voltage, the diode is not activated (does not conduct) and all the current flows through the load. (try to weaken the source)
Also try making the load heavier (push momentary button) and observe a similar effect.
http://www.electronics-tutorials.ws/diode/diode_7.html
check also my other examples http://everycircuit.com/circuit/5224206555414528
