Philosophy begins with the search for principles. Thales spoke of water as the origin of all things, Anaximenes spoke of air. Empedocles thought that fire, water, air, and earth were mixed by Love and separated by Strife, and Plato gave us the Forms.
When Aristotle approaches the subject of principles, he does so in a different manner than his predecessors. He is answering Parmenides' doubts about change, and in so doing he establishes the true principles of change and of things themselves.
In all change, something goes from something to something else. The something else is the "to which" and the original something is the "from which." Consider a man becoming musical. A man goes from being unmusical to musical. There is a real sense in which the "unmusical" is a type of non-being, so there is a problem. It is impossible that something (musical) should come from nothing (unmusical).
Aristotle's answer is thus: There are actually three principles of change. Unmusical cannot become musical of itself just as red cannot become blue. The privation, unmusical, does not acquire the musical form, it must inhere in something else. In our example, man would be the underlying subject of change. Unmusical man becomes musical man.
For Aristotle, the privation is non-being per se and matter is only non-being per accidens. Matter is the potency that is able to receive the act of form in order to constitute a material substance. Matter, privation, and form are thus the principles of change.