Who is Apasmara and why is Shiva as Nataraja dancing over him?
Shiva is often pictured doing his Tandava dance of destruction. He is seen with four hands and one leg up, as an expert dancer, and one leg dancing on a small person called the Apasmara-purusha. In two of his hands he holds the damaru drum and fire. The drum represents sound which is supported by ether. This is a sign of further creation after the annihilation or destruction. Fire represents the Pralayagni, or the fire of universal destruction. Thus, Shiva holds the symbols of cyclical universal creation and annihilation. The other two hands represent protection and blessing for those who take refuge of him or of his spiritual knowledge.
The Apasamara upon whom Shiva stands symbolizes the ignorance which make us lose our clarity and consciousness of our real identity. This also signifies our succumbing to the process of death without spiritual preparation.