Concupiscence is an effect of original sin which created a proclivity in our human nature to sin.
It is a spontaneous movement of our sensual appetite toward what we imagine as pleasant and away from what we imagine as painful. Concupiscence of the eyes is an unreasonable desire to see, hear, and know what is harmful to our virtue, inconsistent with our state in life, or detrimental to our higher duties.
Concupiscence of the flesh is when sensual pleasure is desired as an end unto itself, apart from its divinely intended purpose: to facilitate our practice of virtue and to satisfy our legitimate desires.
Concupiscence explains why it is often easier to sin than to practice virtue or seek holiness. It is not an excuse for sinful behavior but explains why we often commit the same sins over and over again, no matter how hard we try to do otherwise. Concupiscence can only be overcome by grace and mortification (penance).