The data codex is an artifact of the ancient Bion race in Metroid Prime: Federation Force. It is a complete repository of the race's knowledge, and was paired with the Orb. A backup copy of the codex was kept in a separate location from the original, with the backup's location being the same as an intended rescue site for the ancient race.
During M02: Hardball, the Space Pirates stole the data codex from its original location on Bion, shortly before the Federation Force was able to find it. The Force is able to recover the Orb itself however, and the Galactic Federation research team quickly discovers it contains a map of Bion with the location of the backup copy. The Force recovers the backup codex during M05: Excavation and brings it to the G.F.S. Aegis safely, fending off an attack by the Space Pirates who had hoped to recover the backup copy as well. The artifact contained a detailed map of Bion that showed the rescue site the ancient race had intended to use, at which a backup copy of the codex could be found.
After M09: Blender, the research team determines from biometric data on the Orb and data codex that the ancient race was composed of giant beings, although their size was artificially increased. The Space Pirates use the race's knowledge to create similar technology, the Amplification beam. This is later used during M22: Convergence to drastically increase the size of Samus Aran, who the Pirates captured and brainwashed into fighting the Federation Force.