When we are merciful we are kind or forgiving toward someone vulnerable, perhaps someone over whom we hold power. Mercy lets us set aside questions of equity, justice, vengeance, or even blame; it asks not what a person deserves, but what they need.
Understanding the brokenness of other people and the pressures they experience makes it easier to be merciful. Mercy is intimately tied to compassion and empathy.
Without mercy, the world would be a harsh, exacting place. When we recognize the grace we have received -- all of the times that we have, mercifully, not gotten what we deserved--we often find in ourselves a desire to give others the same gift.