Matthew 18:21
Then Peter having come near to him said, "Sir, how often shall my brother sin against me and I forgive him...till seven times? Jesus saith to him, "I do not say to thee seven times, but till seventy times seven."
Once Jesus shares this lesson in forgiveness with Peter He begins to tell a parable of an unforgiving servant. This person was indebted to their master big time - like he was in for what could be considered hundreds of thousands of dollars. He didn't have the money to pay so the master would not forgive the debt, but instead commanded he be sold along with his wife, children, and all belongings which was the custom, unless the master had decided to forgive.
Now, although this is a parable it was greatly relatable. Even today it is well understood...this guy had a debt and he had absolutely no way to pay it back. The correlation is that as human beings we were born into sin. From birth we were already guilty because of the fact that we knew the difference between good and evil and as soon as we got the chance we were going to choose one or the other. Unfortunately, there is not a single one of us who at some point hasn't chosen the evil. We owe the Creator for this transgression, but we can't truly pay it back.
In the old testament the Hebrews used to make sacrifices to be forgiven, but they became so routine and rotten that they were completely ineffective for the forgiveness of sin in God's eyes. So, He sent Jesus as the final sacrifice.
John 3:16
For God so loved the world that He gave His only son that those who believe in Him may not perish, but have eternal life.
This guy who helped all sorts of people, gave of himself continually by healing and setting people free was beaten, mocked, and hung on a cross to die in shame so that we would be able to have relationship with the Father and salvation through the shedding of His blood. So, He forgives us. What an awesome thing to not have that debt to pay. Well, it gets this good for the servant as well. Feeling great remorse, the servant bows down to the master and begs him to give more time so he can pay it all back. Out of compassion he simply let him go. He forgave all the debt and allowed him to go on.
Unfortunately, this servant did not possess the same benevolent heart because when he came across one of his own servants who owed him like...twenty bucks, he demanded it back. When the servant asked forgiveness he refused to forgive and had him thrown into prison. Well, word got out and the master ended up finding out what he did. He called his servant back and lifted the forgiveness, making him pay back every cent of what he owed.
After telling the parable Jesus lets the disciples know that Yahweh will deal with them the same way if they do not forgive others in the same way that they have been forgiven. So often though, this is very difficult. We feel justified in our anger and dignified in wrath against someone who has wronged us. That emotion fills us with pride and the last thing we want to do is become humble and forgive. We must though. When we think about it we've done many things in our lives that need forgiveness so holding back on it and not giving it to someone else is just not worth it. Let it go. Wipe it out of your mind and hold nothing against them. Forgive and you will be forgiven. Also, forgive since this is just good medicine for your own heart.