“Pralaya is related to the Indo-European root “przerwa” or “pauza” or pause - it is the pause between yugas when the loka are destroyed.”
In other words it is the root substance of the universe when the universe goes into (acccording to easter sources) negative existance. It is the source of everything when everything goes out of negative existence. In other words it is the root substance of the universe, as I said.
No, it’s not the root substance as per Santana dharma. That is Eshwara