An Armature is a supporting skeleton usually made of thick gauge wire. It serves as both practical support for the weight of the sculpting material and an anatomical skeleton. Creating the armature the right size can help ensure that the sculpt is the correct scale. The armature is sometimes wrapped in more wire or in aluminum foil to bulk it up in places where the sculpt would be thick.
Creating a sculpt without an armature can lead to use of far too much material and a overly heavy final product. This can also lead to inadvertent air bubbles and cracks when baking the piece.