If grace is getting what you don't deserve, then mercy is NOT getting what you DO deserve. It's like 2 sides of the same coin - God's amazing grace and His tender mercy.
When God looks down upon His fallen creation, He has mercy on those whom He has created. Though we deserve nothing but punishment and death because of our sin, He shows mercy to us because of the suffering and death of His Son Jesus Christ on our behalf. Instead of punishing us and condemning us for our sin, He offers us His love, mercy, compassion, and forgiveness. When we believe in Him, when we put our faith and trust in Jesus and receive His forgiveness and salvation, we are not only freed from the chains of sin and death, we are also freed up to do good works, and to show mercy to the people around us who need it.
Because God has shown this tender mercy to us, we strive to do the same for our neighbor. As God's loved and adopted children, we are called to treat people with dignity, kindness, gentleness, and respect.
And as we go about our new missions of mercy as free and forgiven children of God, this short little seven-word prayer, spoken by the tax collector in one of Jesus' parables, is one of the best prayers we can ever say - "Lord, have mercy on me, a sinner."