The Over-Soul refers to the unique mechanic of Caledonia and the drow people. When drow die, their soul is taken into the Over-Soul around the planet and will then be reborn. This includes all life on Caledonia. This essentially makes the drow immortal as they will, eventually, be reborn after death.
When born again, a drow has no old memories. The memories of their past lives will only begin to resurface during their Awakening in their teenage years. Memories will return, piece-by-piece, including skills and even their true name[Pan 2].
"The name of the Over-Soul was inspired by the term used by Ralph Waldo Emerson[Ext 1] to describe his own over-soul[Ext 2] concept. The actual mechanic has nothing to do with the inspirational source of the name, however." ~ Britt the Writer