continue at Unam Sanctam Catholicam
Pope Pius XII gave us a masterfully concise definition of the devotion in his encyclical Haurietis Aquas:
"Devotion to the Sacred Heart of Jesus, by its very nature, is a worship of the love with which God, through Jesus loved us, and at the same time, an exercise of our love by which we are related to God and to other men" 
The holy pontiff wrote these words at a time when devotion to the Sacred Heart was being attacked by the progressives as unbiblical, as inappropriate for the modern day, and as constituting an inappropriate exaltation of the human nature of Christ. As we will see, the venerable Pius XII was entirely correct in insisting on the theological and practical fittingness of this devotion for the modern age.