The play of life and death expresses Her activity in Nature: creation, preservation, and destruction. Hence the sword, the head, and a third hand extended, bestowing life.
“Her energy is omnipresent; hence Her streaming hair, representing energy.
Shiva, Her husband, represents God in His vibrationless state, beyond creation. Thus, He is depicted as supine.
Basically that Kali being the Shakti, creates, preserves and liberates the Universe. All this She does on behalf (because they are both One) of Lord Shiva, who is the vibrationless, Nirguna Brahman, motionless, undescribable, bliss and without attributes. Thus Shiva lays motionless and smiling in bliss, while God Kali performs the acts on His behalf.
"Whatever you perceive in the universe is the outcome of the union of Purusha and Prakriti. Take the image of Shiva and Kali.
Kali stands on the bosom of Shiva; Shiva lies under Her feet like a corpse; Kali looks at Shiva. All this denotes the union of Purusha and Prakriti.
Purusha is inactive; therefore Shiva lies on the ground like a corpse.
Prakriti performs all Her activities in conjunction with Purusha. Thus She creates, preserves, and destroys.
Thus Kali represents the vibration or Aum which is responsible for creation, preservation and liberation, while Shiva is the absolute."
~~ Sri Ramakrishna Paramhansa!