Yellow Lums are the 1000 fragments of the Heart of the World broken by the pirates. They contain universal knowledge. The more you gather, the more you will learn. You can access the secrets of the world by pressing F1.
There are 4 types of Lums. The most important ones are the Yellow Lums. There are 1000 of them. The Yellow Lums give knowledge to those who gather them and can also open doors into unknown places...
These are the 1000 shards broken from the Primordial Core when the pirates exploded it. When Rayman has collected enough of them, he'll be able to negotiate his entry into new worlds. In addition, they contain precious knowledge. The more Rayman can gather, the more he knows of the secrets of the world. At any moment in the game, just press F2 to read new information provided by the Yellow Lumz.—Manual (PC version), Rayman 2: The Great Escape
These are the 1,000 shards broken from the Primordial Core when the pirates exploded it. When Rayman has collected enough of them, he’ll be able to negotiate his entry into new worlds.—Manual, Rayman 3
These Lums arrange themselves to guide Rayman and Globox along the safest path through the level; they also do their best to increase Rayman and Globox's score.—Manual, Rayman: Hoodlums' Revenge
Yellow lums are pieces of the Primordial Heart that the rabbids have blown up. Once Rayman has collected enough, he can access other worlds.
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 for the first time in the Fairy Council in Rayman 3. The Game Boy Advance version of Rayman Raving Rabbids contradicts the previous games regarding the number of Lums, featuring 1550 of them to collect, rather than the previously established 1000.
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 first appeared in Rayman. If enough Yellow Lums are collected, the collector can find out the Knowledge 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 and Rayman no longer receives the Knowledge of the World from them. In this version, the Yellow Lum which Razorbeard eats is replaced with a Red one. There are also 800 Lums in Rayman 2 Forever. 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. However, in the Nintendo DS version the Yellow Lums have a totally different use; Rayman uses Yellow Lums to throw Energy Spheres, which do greater damage on enemies. Yellow Lums can be found in a great group, being called as "Bag of Lums". These are hidden in a single unit for every level of the game.