From my understanding, Aristotelian metaphysics uses the concepts of potentiality and actuality to explain change. Since being cannot come from non-being, when change occurs a potential is simply being actualized. However, what exactly is a potential, metaphysically speaking? What kind of substance is a potential, and what happens to it when it is actualized?
Potentiality is not a substance. Substances are matter+form composites.
As St. Thomas Aquinas writes in his short word On The Principles of Nature 5.,
everything which is in potency can be called matter
omne quod est in potentia potest dici materia
Matter is a mode of being midway between non-existence and substance.