Selection from Edward Feser, Aquinas: A Beginner's Guide, (London: Oneworld, 2009), 9.
Parmenides assumed that the only possible candidate for a source of change in a being is non-being or nothing, which (of course) is not source at all. Aristotle's reply was that this assumption is simply false. Take any object of our experience: a red rubber ball, for example. Among its features are the ways it actually is: solid, round, red, and bouncy. These are different aspects of its 'being.' There are also the ways it is not; for example, it is not a dog, or a car, or a computer. The ball's 'dogginess' and so on, since they don't exist, are different kinds of 'non-being.' But in addition to these features, we can distinguish the various ways the ball potentially is: blue (if you paint it), soft and gooey (if you melt it), and so forth.
So, being and non-being are not the only relevant factors here; there are also a thing's potentialities. Or, to use the traditional Scholastic jargon, in addition to the different ways in which a thing may be 'in act' or actual, there are the various ways in which it may be 'in potency' or potential. Here lies the key to understanding how change is possible. If the ball is to become soft and gooey, it can't be the actual gooeyness itself that causes this, since it doesn't yet exist. But that the gooeyness is non-existent is not (as Parmenides assumed) the end of the story, for a potential or potency for gooeyness does exist in the ball, and this, together with some external influence (such as heat) that actualizes that potential--or, as the Scholastics would put it, which reduces the potency to act--suffices to show how the change can occur. Change just is the realization of some potentiality; or as Aquinas puts it, 'motion is the actuality of a being in potency' (In Meta IX.1.1770), where 'motion' is to be understood here in the broad Aristotelian sense as including change in general and not just movement from one place to another.