Significance of Kali standing on top of Shiva
Shiva is the transcendental state; it means one who brings only benevolence and good to the world.
So when Kali, an enormous, destructive power was in rage; in that moment, Shiva came and lay down. As soon as she stepped on him, Kali’s power got controlled, channelized and became benevolent. So, even if there is destruction, it should be benevolent.