Yellow Lums are the most important of all Lums. They, along with the Red, Blue, and Purple Lums, created the god Polokus, who went on to create the Glade of Dreams. There are exactly 1000 Yellow Lums in existence; when united (which seems to be their natural state), they form the Heart of the World, which was shattered into its constituent Yellow Lums by the Robo-Pirate army prior to the beginning of Rayman 2. Rayman collected all 1000 of them throughout the course of this game; the Heart (which contains them) is seen again in the Fairy Council in Rayman 3.
There is some evidence to suggest that the Great Protoon and the Heart of the World are in fact the same object; the reason for its differing appearance in Rayman and Rayman 3 may be its destruction by the Robo-Pirates and its subsequent reconstruction by Rayman and the Teensies. If this is true, then the 1000 Yellow Lums (in the form of the Great Protoon) actually appeared in Rayman. If enough Yellow Lums are collected, the collector can find out secrets of the world.
In most versions of Rayman 2, Razorbeard eats a Yellow Lum during a cutscene. This causes the total Yellow Lum counter on the heads-up display to decrease from 1000 to 999. This Lum, however, can later be found in a secret room in the Tomb of the Ancients. In Rayman Revolution, it is instead found in the same chest as the Lums Radar, also in the Tomb of the Ancients.
In the PlayStation version, there are only 800 Yellow Lums, which do not give Rayman the Knowledge of the World. In this version, the Yellow Lum which Razorbeard eats is replaced with a Red one. There are also 800 Lums in Rayman 2 Forever and when the word Lums first comes up, it says it as ‘Lumz’.
In the Game Boy Advance version of Rayman Raving Rabbids, there are 1550 Yellow Lums to be found.