It's a type of diode that allows current to flow in one direction only, and it has the special property of conducting in the reverse direction when the voltage reaches a certain level (called the breakdown voltage).
In this case, the Zener diode has a breakdown voltage of 18V.
The 1/2W refers to the power rating of the diode, which means it can safely dissipate up to 1/2 watt of power.