Rappelling or Abseiling is the skill of descending down a rope. It is actually a simple process, but is fraught with many hazards, some statistics state that about 25% of climbing deaths occur during rappelling, most commonly due to failing anchors or abseiling beyond the end of the rope.
At its core, rappelling is the act of using some sort of friction device or hitch to play out your rope in controlled fashion, under load, so that you can safely descend.
Rappelling is not rocket science, but you have to know what you are doing. The rope has to be properly anchored, you have to be properly attached to the rope and the device you are using to rappel with needs to be properly attached to YOU.