An object of legend, it is said to hold the knowledge of all robot based technology in the Galaxy.
Legend holds that the Compendium is the library of Maxion Prime, the 'god of Super Robots' and holds all the secrets of his technology. It is unknown who started the rumor.
Because of this, the Compendium is sought out by numerous robotic races and organic races, all of whom desire the technological secrets within. This has lead to a sort of race by several powers in the galaxy to find and acquire the Compendium.
The Compendium is another name for the Regenesis Matrix, which became a thing of legend and was sought after by various parties for the knowledge held within. Because of the mysterious nature of the Matrix, no knows for sure what it is, some think it is a book, a data core, or even a key to a library of sorts.
It is believed to have information on robotics, Weaponry, defenses, computers and the biology of Robotic Lifeforms. Basically, if it can be put on a Super Robot, then it's on the Compendium.