The Catechism in Questions & Answers

638. What does it mean to pray: “And forgive us our debts, as we forgive our debtors”?

All human beings incur guilt on account of their sins. With these words, the believers acknowledge that they are sinners before God, and ask Him for forgiveness. Because God is gracious and forgives us, He expects us to likewise forgive those who have wronged us. Therefore, we will only be granted forgiveness if we ourselves are reconcilable and willing to forgive.

“Then Peter came to Him and said, ‘Lord, how often shall my brother sin against me, and I forgive him? Up to seven times?’ Jesus said to him, ‘I do not say to you, up to seven times, but up to seventy times seven’.”
Matthew 18: 21-22