What is this Devotion?
We know what it means to be a devoted mother, father, or child. Devoted people are those who love, those who care, those who are generous of themselves in loyalty to a person, group, or cause. When we speak of devotion to the Sacred Heart we refer to the type of veneration in which we worship the person of Jesus Christ, symbolized by his heart. Every devotional practice approved by the Church is based on a doctrine (a revealed truth of faith). The devotion of the rosary, for example, is founded on the doctrine that Mary is the Mother of God, otherwise, we would not pray the rosary in her honor. 
The doctrine on which devotion to the Sacred Heart rests is in this quotation from the Bible, "God is love, and whoever remains in love remains in God and God in him" (1 John 4:16). Since Jesus is God and the heart is a symbol of love, we are honoring the person of Jesus in this devotion. Pictures and statues of the Sacred Heart should remind us of how much God loves us and how much we should love him in return. 
The symbols surrounding our Lord's wounded heart are explained as follows. The crown of thorns is around his heart and not his head to indicate that his immense love for humanity has been the cause of his suffering. The cross implanted in his heart reminds us of his interior sufferings - the humiliations, sorrows, and contempt he submitted to even before he died on the cross. The wound in his heart was made by the centurion's lance and signifies in this human way what sin does to him. The flames indicate the intensity of his love for all people. All these symbols point to a direct relationship between devotion to the Sacred Heart of Jesus and the Passion of our Lord; hence, the importance of the Stations of the Cross, night adoration in church, and the Holy Hour in the home. 
One of the characteristic elements of this devotion is reparation, which has to do with repairing or making up for injuries or wrongs done. Our Lord asks this of his faithful friends to console him for the coldness and indifference of so many. This is done by receiving Holy Communion on the First Fridays, praying the Holy Hour (in the Church and in the home), and, most importantly, by striving to love him more in order to make up for those who do not love him. Think of your own faults and failings. Now think of the many outrages committed by others throughout the world. God forgives sin when true sorrow is shown, but how many sins are not repented of? How much penance have people done through the centuries? Reparation means that we willingly do something extra, make some sacrifice to answer for our sins and those of the whole world.

Source: Fr. Christopher Farrell, C.SS.R.