According to the Catechism of the Catholic Church, "one commits venial sin when, in a less serious matter, he does not observe the standard prescribed by the moral law, or when he disobeys the moral law in a grave matter, but without full knowledge or without complete consent. Venial sin weakens charity; it manifests a disordered affection for created goods; it impedes the soul's progress in the exercise of the virtues and the practice of the moral good; it merits temporal punishment. Deliberate and unrepented venial sin disposes us little by little to commit mortal sin. However venial sin does not break the covenant with God. With God's grace it is humanly reparable. 'Venial sin does not deprive the sinner of sanctifying grace, friendship with God, charity and consequently eternal happiness.' While he is in the flesh, man cannot help but have at least some light sins. But do not despise these sins which we call 'light': if you take them for light when you weigh them, tremble when you count them. A number of light objects makes a great mass; a number of drops fills a river; a number of grains makes a heap. What then is our hope? Above all, confession...."
Catechism of the Catholic Church