Divine Mercy is the good news that God Himself offers each one of us His mercy - His forgiveness - for all our sins! It's a fantastic message that Jesus Christ brought to a nun in Poland, Sr. Faustina, during the 1920's. Jesus asked her to have a painting made of Him, with the rays of Mercy emminateing from His heart to the person looking at the painting. This is called the image of Divine Mercy and is how Jesus appeared to Sr. Faustina.
Jesus also gave a special set of prayers to Sr. Faustina and asked people to pray these prayers to intercede for the sins of others. These prayers are known as the Divine Mercy Chaplet and are recited on standard rosary beads. Jesus asked that they be said at 3 PM which was the time He died on the cross for our sins.