As the Japanese name implies (空想具現化 Kuusou Gugenka), Marble Phantasm is the ability to materialize a phantasm -- visual genjutsu.
It is the ability of a powerful being capable of manipulating the very fabric of reality to connect their will to nature, to interfere on probabilities, and to transfigure the surrounding world at will according to their vision of the world. As the user is a part of nature, he or she can change the world as desired. The capacity of the actualization is proportional to the scale of the user's potential. A transformation independent from nature (such as transforming a human) cannot be realized, unless done indirectly.