"<3" is the ASCII/Emoticon representation of a heart.
Although <3's is a heart, it isn't like an anatomically correct heart, nor a heart symbol, instead looking like a combination of the two.
According to Edmund McMillen's first post-Repentance Lore Stream, the visual appearance of this item is a reference to the movie 'Raising Arizona' in which the character of Glen passingly remarks about an orphan child with his heart born on the outside of his chest.
McMillen states that this remark left a lasting impression on him when he was younger, and as such he wished to incorporate it into the game in some way.
This scene itself is a reference to the rare birth defect 'ectopia cordis' in which the child's heart is located either partially or entirely outside of their chest.
With Isaac canonically being 3 apples tall, the amount of body covered by the heart is fairly accurate compared to the size of an average child heart.
Soul Hearts and 
Black Hearts.
Gluttony.
Tainted Bethany: Grants 12 blood charges.
The Binding of Isaac: Rebirth
