Today is the Feast of Saint Margaret Mary Alacoque, who is responsible for the spread of the devotion to the Sacred Heart of Jesus: The bleeding heart represents love without holding back. The Thorns around the heart represent total sacrifice, and the fire represents passion. There is also the cross that is on the heart itself, for ALL these things come together when Jesus suffered on the cross.
Here is a link to a prayer to the Sacred Heart of Jesus- a heart that loved us without holding ANYTHING back. May we live our lives with the same love, passion, and sacrifice.