Rez in Second Life means to create or to make an object appear. Rezzing an object/prim can be done by dragging it from a resident's inventory or by creating a new one via the edit window. The term "rezzing" can also be used for waiting for a texture or object to load, such as "Everything is still rezzing." or "Your shirt is still rezzing for me."
Taken from the movie Tron's term "de-rezz", which roughly means to dissolve in a certain way.
Example LSL Script
touch_start(integer num_detected)//Touch event
llRezObject("Object",llGetPos()+<0,0,2>,<0,0,0>,<0,0,0,0>,0);//This will rez the object named "Object" 2m above the object this script is in.