Why is the altar stone covered in linen?

The "altar stone" is the true altar. The rest of the "altar" is not strictly speaking an altar, but the support for the altar. The altar stone usually has the relics of saints contained within it, and it should be covered with three clothes, although nowadays, many places dispense with the lower two. Symbolically, the linen cloth (the topmost cloth that covers the altar) signifies the linen in which the dead body of Our Lord was wrapped.