The basic idea is really not all that complicated, and can be understood at least to a first approximation by reference to everyday examples. Everyone knows that it is in the nature of grass to require water and sunlight but not too much heat, and that for that reason it is good for grass to be watered and well lit and bad for it to lack water and sunlight or to be exposed to great heat. Everyone knows that is in the nature of a tree to require soil into which it can sink its roots and from which it can draw water and nutrients, and thus that it is good for a tree so to sink them and bad for it if it is somehow prevented from doing so. Everyone knows that it is in the nature of a squirrel to gather nuts and the like and to dart about in a way that will make it difficult for predators to catch it, and thus good for it to do these things and bad for it if for whatever reason it fails to do them. The natures of these things entail certain ends the realization of which constitutes their flourishing as the kinds of things they are.
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Now, none of these examples involves moral goodness or badness, because morality involves intellect and will, which grass, trees, and squirrels all lack. Rational creatures like ourselves are capable of moral goodness or badness precisely because we do have intellects and wills. The will itself has as its natural end the pursuit of the good, and determining what is in fact good is part of the natural end of the intellect. Morally good action thus involves the will to do what is good for us given our nature, while morally bad action involves willing contrary to what is good for us given our nature. And to the extent that the intellect knows what is good for us we are culpable for these good or bad actions. To will to do what is “natural” for us thus means, in classical natural law theory, something like to will to do what tends toward the realization of the ends which, given our nature, define what it is for us to flourish as the kind of things we are. And to will to do what is “unnatural” thus means something like willing to do what tends toward the frustration of the ends which, given our nature, define what it is for us to flourish as the kind of things we are.
