It's easier explained with the example of debt. You owe something. The person you owe can choose to forgive you which means, they saying you no longer owe. That's the concept, but obviously, it applies no just in the context of money.
Now you can't really forgive someone who thinks they didn't cause you harm in the case where they truly did, and neither can you, on behalf of another person, forgive someone else who didn't cause harm to you, rather to another person. But it's kinda part of our own debt (aka we "supposed to") to forgive people when they admit when they messed up, who want your forgiveness, and who also had an apparent change of heart about whatever offense they caused to you. Whether that helps you sleep at night or not is a matter ofconscience. But if you talking about morally, then yes you supposed to based on the golden rule.