Read: Matthew 6:14, 15
For if you forgive others their trespasses, your heavenly Father will also forgive you, but if you do not forgive others their trespasses, neither will your Father forgive your trespasses. (Matthew 6:14, 15 ESV)
We have all been sinned against. We have all sinned against God, and other people. The English Standard Version uses the word trespasses which paints a significant word picture. All of us have passed into territory where we do not belong.
Thinking in terms of trespassing the idea of separation from God takes on a lot more meaning. God is sovereign. He is the rightful king of our hearts. But, when we assert selfish control over our own lives we have trespassed into His territory. This is the essence of sin and it puts distance between our hearts and our holy God.
Likewise people trespass into our territory. Perhaps they do so accidentally, or maybe even with pure intentions, but it is inevitable that we will be wronged by others at some point. This creates distance and discord between our hearts. Jesus taught that it is his followers' duty to forgive those kinds of trespasses committed against us. We are to extend grace to our fellow man when that situation occurs.
Trespassing happens because of the idea of ownership. We easily, willfully, and frequently trespass against God because we think we possess ownership, when in fact He is in total ownership. We regularly feel trespassed against for the same reason. Sometimes it is valid as others extend their perceived ownership too far. Often it is invalid as the pride created because of our false sense of ownership is offended. Even this is encroaching on God's dominion.
Forgiveness happens, and can only happen, when we lay aside our false sense of ownership over places in our hearts. When we are unthreatened by someone's presence somewhere we will gladly welcome them in. This action extends forgiveness to others, but it also helps us to receive forgiveness from the Father.