Source codes, or codes for short, are pieces of the Annex Program in Code Lyoko Evolution. X.A.N.A. implanted them in each of the Lyoko Warriors (minus William and Jeremie) before it was destroyed in Fight to the Finish. The codes are pieces of what makes up X.A.N.A., and he tries to get them back from the Warriors using spectres sent to Earth to drain the codes. If X.A.N.A. manages to get all the codes back, it will be able to completely take over the entire network.
The codes allow users to access towers and various interfaces. In the Evolution timeline, only people with source codes can enter activated towers (but they can enter Way towers without them), and they can also deactivate them, unlike in seasons 1-4 when only Aelita and X.A.N.A. had this ability. As long as they still have source codes inside them, Ulrich, Odd and Yumi are all capable of deactivating towers.
In first episodes Ulrich, Odd and Yumi lost a lot of codes and X.A.N.A tricked Aelita with specter that resembled her mother to get most of codes inside her.
Odd was first one to loose all the codes in The Codeless. Jeremie managed to transfer few percents of codes back to him eventually, but he lost them again in Massacre with Ulrich loosing all of his soon after. Yumi and Aelita still possess some of the codes inside themselves and X.A.N.A had drained 95% of the codes.