The Heart of the World, also known as The Primordial Energy Core, is the ultimate source of life in Rayman's universe.
Admiral Razorbeard and his Robo-Pirates destroy the Heart of the World and breaks it into 1,000 lums (800 in the PS1 version). Rayman must recover each and every lum to fully restore the heart.
In Rayman 3, Andre's main goal is to destroy the Heart of the World. Unlike the Robo-Pirates, he never succeeds, but gets fairly close. In this game, it is revealed that The Teensies like to hang out and party at the Heart of the World. Apparently there is a secret ledge on a pillar which leads to a "freak-out" slide with rubies, yellow gems and an emerald.