The Zener diode acts like a one way valve for electricity just like normal diodes. Like normal diodes the 'valve' will give way and allow electricity to flow the other direction when the voltage is high enough. Unlike other diodes though, the Zenor diodes is intended to break down. The Zener voltage of 5.1Volts in this case is the point at which this break down occurs. This is in a useful range for doing interesting things in a circuit like producing a reference voltage for providing power at a known voltage and low current.