What is venial sin?Venial sin is a less serious offense against the law of God, which does not deprive the soul of sanctifying grace, and which can be pardoned even without sacramental confession.
Be ye therefore perfect, as also your heavenly Father is perfect. (Matthew 6:48)
How can a sin be venial? A sin can be venial in two ways:
How does venial sin harm us? Venial sin harms us by making us less fervent in the service of God, by weakening our power to resist mortal sin, and by making us deserving of God's punishments in this life or in purgatory.
But I tell you, that of every idle word men speak, they shall give account on the day of judgment. (Matthew 12:36)
Further reading: CCC 1862-1864