Précis is a skill which involves reducing a text to fewer words to obtain the gist or essential point. In a sense, an essay is a précis of all you know about a subject. Try a précis of the following extract which explains Aquinas’ theory of natural law in 100-120 words.
Source: http://www.aquinasonline.com/Topics/natlaw.html

Aquinas asks whether there is in us a natural law. First, he makes a distinction: A law is not only in the reason of a ruler, but may also be in the thing that is ruled. In the case of the Eternal Law, the things of creation that are ruled by that Law have it imprinted on them through their nature or essence. Since things act according to their nature, they derive their proper acts and ends (final cause) according to the law that is written into their nature. Everything in nature, insofar as they reflect the order by which God directs them through their nature for their own benefit, reflects the Eternal Law in their own natures. (S.T. I-IIae, 91, 2)

The Natural Law, as applied to the case of human beings, requires greater precision because of the fact that we have reason and free will. It is our nature to act freely (i.e. to be provident for ourselves and others) by being inclined toward our proper acts and end. That is, we human beings must exercise our natural reason to discover what is best for us in order to achieve the end to which their nature inclines. Furthermore, we must exercise our freedom, by choosing what reason determines to naturally be suited to us, i.e. what is best for our nature. The natural inclination of humans to achieve their proper end through reason and free will is the natural law. Formally defined, the Natural Law is humans' participation in the Eternal Law, through reason and will. Humans actively participate in the eternal law of God (the governance of the world) by using reason in conformity with the Natural Law to discern what is good and evil.

In applying this universal notion of Natural Law to the human person, one first must decide what it is that God has ordained human nature to be inclined toward. Since each thing has a nature given it by God, and each thing has a natural end, so there is a fulfilment to human activity of living. When a person discovers by reason what the purpose of living is, he or she discovers his or her natural end is. Accepting the medieval dictum "happiness is what all desire" a person is happy when he or she achieves this natural end.