It's a word I can't say I hear much. I never heard it at any time during the nine years I attended Catholic schools. Very few priests in my neck of the woods have ever uttered it from the pulpit, either. Were it not for EWTN, the Internet and my voracious reading habit, I might have never run across this word ever. Yet it's of the utmost importance to a Christian working out his or her salvation.
Concupiscence, which literally means the state of having desire, is what the Catechism of the Catholic Church calls "an inclination to sin" (CCC 1264). It's our desire to gratify our senses, sometimes without first running them through the filters of good reason. How can we master our desires if we do not understand our desires and how they properly fit into God's plan? (This is why I'm so puzzled that I've not heard the concept of concupiscence discussed more often. There is some really great stuff out there, though; for example, click here for Fr. Robert Barron's compelling sermon on the woman at the well.)
Human beings are quite complex, obviously. We have appetites and sensual desires of all kinds. Our want for food, drink, sex and things of all kinds are rooted in our drive to sustain ourselves as individuals and as a human family. Because our desires are a part of our makeup as human beings created by God, our desires are actually good things. The wherefores should be rather obvious.
It's when our desires go unchecked, when satisfying our passions supersede the good of humanity and our obedience to God that we are in trouble. This is the heart of Catholic teaching on concupiscence: our sensual desires, while in and of themselves are not evil, can be incentive to sin, especially when sensual desire becomes the object of our happiness rather than our desire for God. Mastering these sensual desires are an integral part of working out our salvation.
A line from the Act of Contrition comes to mind: "I firmly resolve with the help of Your grace to sin no more and to avoid the near occasion of sin." I believe this is why St. Josemaria Escriva encourages us to scorn concupiscence--not because it is sinful, but because we are.
From New Advent:
I find then a law, that when I have a will to do good, evil is present with me. For I am delighted with the law of God, according to the inward man: but I see another law in my members, fighting against the law of my mind, and captivating me in the law of sin, that is in my members. Unhappy man that I am, who shall deliver me from the body of this death? (Romans 7:21-25)
Click here to read more about concupiscence at New Advent's Catholic Encyclopedia.