The message of mercy is that God loves us — all of us — no matter how great our sins. He wants us to recognize that His mercy is greater than our sins, so that we will call upon Him with trust, receive His mercy, and let it flow through us to others.
You can read the background of the Divine Mercy devotion here. The novena begins today, Good Friday, in order to end on Divine Mercy Sunday. Will you pray it this year?