Rappelling double strand is just like it sounds; you rappel down two strands of a rope. There are lots of situations where you might want to rappel double strand.
Rappelling double strand gives more friction than rappelling single strand. The added friction gives added safety and more control, in most situations.
Sometimes the added friction of having two ropes through your rappel device makes it so light rappellers get stuck on rappel or have a very hard time getting down.