The screen or decoration, often carved or including statuary, which is behind the altar of a church, built into the wall. Originally worship in churches was “oriented”—worship faced to the East, and the reredos was thus on the East wall with the altar built immediately in front on the wall and attached to the wall. The priest faced the altar (i.e., away from the people), with the idea being that all people faced in the same direction, East, from whence Jesus is to return at the second coming. (This is why graves face East.) Because all the people were facing East, this wall became more and more decorated to orient the people to worship.
The word reredos comes from areredos, which is Anglo-Norman and derives from arere (behind) and dos (from the Latin dorsum or “back”).