Concupiscence is a very important concept in examining the origin, cause, and cure of sin.
It is an uncommon word, but the concept is simple. It has two meanings. One is the very broad meaning of the overall desire for good. This is, obviously, good. However, the reason why this word exists is because of the disorder caused by Original Sin. The specific meaning which concerns us is the post-Fall state of the lower desires of the flesh being contrary to reason. A simple and well known example would be the urge to eat or consume something which one knows is potentially harmful. The flesh is rebelling against reason in that it is drawn towards what is good, but without reason. Concupiscence is what we all have from birth until death. When the grace of God is restored to us, concupiscence remains like a scar from a healed wound.
This disordered aspect of concupiscence was not the design of God. It is the result of sin, the removal of what is good.
While one does not need a theological study of this or a complete understanding, it is essential to understand that this exists and that it leads to sin.
And the final trap is becoming lax. We may have chosen truth, we may have sanctifying grace, and we may be avoiding all proximate occasions of sin, but we still have the wounds of our generation and we must always actively guard against them because if we do not, we will be dragged down through our own fault.
Lent, Advent, and other times designated for fasting and mortification of the flesh are, a liturgical time of penance, and it is not a ritual or just a tradition from another time, but a real Church given time to fight against concupiscence through fasting and other works of penance.