For me to attempt to explain it would be to rob it of it's power. I will say that for me, it is a reminder of what all our hearts should be and can be thanks to Pentacost event, the outpouring of the Holy Spirit, when God shared his divinity with us.
There are literally thousands of images of the sacred heart, sagrado corazón, sacre coeur, etc. Today we celebraste the heart of Jesus as our highest level of celebration a Solemnity. I would invite every one to go online search for images of the sacred heart, find the one that speaks to you, and loose yourself in God's love.