To forgive someone means to release from liability to suffer punishment or penalty...forgiveness is undeserved and cannot be earned...forgiveness requires that you absorb certain effects of another person's sins and release the person from liability to punishment. This is precisely what Jesus accomplished at Calvary. He secured our forgiveness by taking on himself the full penalty of our sins. Remembering what he did to purchase our forgiveness should be our greatest incentive to release others from the penalties they deserve.
I don't think I've ever understood what forgiveness means...especially in my marriage. I hope that God will grow me in this continually.