Oprah and others, have defined Forgiveness as recognizing that you can't change the past. We hold onto a lot of resentment and in the end, who does it hurt? We hurt ourselves more than anyone else. Accepting what has happened and creating the space to move on can be a powerful step forward.
Sometimes, particularly if you have been a victim of crime or abuse, whatever happened may seem 'unforgiveable'. In these cases, remember that forgiveness does not mean you have to let that person back into your life but it can release you from the hold that this has on you.