The Catechism explains the cardinal virtue of prudence:
§ 1806 Prudence is the virtue that disposes practical reason to discern our true good in every circumstance and to choose the right means of achieving it; the prudent man looks where he is going. Keep sane and sober for your prayers. Prudence is right reason in action, writes St. Thomas Aquinas, following Aristotle. It is not to be confused with timidity or fear, nor with duplicity or dissimulation. It is called auriga virtutum (the charioteer of the virtues); it guides the other virtues by setting rule and measure. It is prudence that immediately guides the judgment of conscience. The prudent man determines and directs his conduct in accordance with this judgment. With the help of this virtue we apply moral principles to particular cases without error and overcome doubts about the good to achieve and the evil to avoid.
Father Hardon explained the virtue of prudence this way:
Correct knowledge about things to be done or, more broadly, the knowledge of things that ought to be done and of things that ought to be avoided. It is the intellectual virtue whereby a human being recognizes in any matter at hand what is good and what is evil. In this sense, it is the moral virtue that enables a person to devise, choose, and prepare suitable means for the attainment of any purpose or the avoidance of any evil. Prudence resides in the practical intellect and is both acquired by one’s own acts and infused at the same time as sanctifying grace. It may be said to be natural as developed by us and supernatural because conferred by God. As an act of virtue, prudence involves three stages of mental operation: to take counsel carefully with oneself and from others, to judge correctly on the basis of the evidence at hand, and to direct the rest of one’s activity according to the norms determined after a prudent judgment has been made. (Etym. Latin prudentia, foresight in the practical order; from providentia, foresight, directive care, providence.)
Mark Shea in the National Catholic Register aims for a more practical explanation:
The Church does not function by the maxim, “That which is not forbidden is compulsory.” Nor does she function by the “Simon Peter Says Principle.” When the Church offers us clear, unambiguous, and authoritative teaching our duty is to give it assent, not to search for loopholes and Clintonian redefinitions of obvious things like “artificial contraception,” “torture,” or “baby.” So unless a case can be made that the Church’s guidance is really going to result in immorality, or is radically impractical, counterproductive, or destructive of human flourishing, our default position is to be obedience, not “How little of the Church’s teaching can I get away with obeying?” In the cases of artificial contraception, abortion, and of torture, no such case exists.
Second Exodus prefers a simpler explanation: Prudent judgment consists in following a well-formed conscience to the highest level of Church teaching in the hierarchy of truths applicable to the instant situation. It follows St. Ignatius of Loyola ‘s maxim, sentire cum ecclesia, “think with the Church,” or more colloquially, “think as the Church thinks.”
(sometimes called “Prudential Judgment”)