The Immaculate Heart is one of the Blessed Mother's most beloved titles. This is not hard to understand because she shares her heart title with her Son, Jesus, as the Most Sacred Heart. The two often appear as the Two Hearts in many painted images.
Look closely at her heart and you will see that it is full of symbolism including:
her heart being outside of her body and she is pointing at it with her fingers;
flames - her heart is on fire with love of us
white roses for her purity
a sword signifies her sorrows
and rays of light remind us that she is the "woman clothed with the sun."
All of that in a single image. Truly beautiful and amazing.