Understanding Forgiveness takes place after you obtain an understanding of the most quoted verse in the Bible, John 3:16 (KJV):
For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.
God giving His Only Begotten Son enables Him to offer you forgiveness for each and every sin that you have committed. Prior to Christ dying for you, the only option that you had was to have your sins accumulate and that He could not accept. God gave you a way out when He had Christ shed every drop of His Blood to place forgiveness within your reach. This sacrifice provided you with one more option. As before you can continue to carry the weight of your sins into the future or you can accept the sacrifice of Christ shedding His Blood to is Blood was wash them away.
1 Peter 3:18 (KJV) shows God's fairness; it shows Him giving this same option to everyone that had died before Christ was put to death in the flesh. Christ went to visit them and give them the same opportunity that you have, the opportunity to be forgiven for sin:
For Christ also hath once suffered for sins, the just for the unjust, that he might bring us to God, being put to death in the flesh, but quickened by the Spirit:
Quickened by the spirit, 1 Peter 3:19 (KJV) shows Christ going back in time to prison where they were being held until He could come and preach faith and forgiveness to them:
By which also he went and preached unto the spirits in prison;
God references where Christ went as prison because it held the ones that had died before the sacrifice of Christ could free them. Without the opportunity for forgiveness they are in prison, an opportunity that Christ returned when He went to them and preached forgiveness. He squared everything by making the same "whosoever believeth" that is available to you, available to them.
Understanding Forgiveness leads to receiving it and receiving it requires that you meet certain conditions. It requires that you:
(1) Ask to be forgiven and
(2) Demonstrate a feeling of remorse for those that you have wronged and