The universal source code was mentioned by the Ice King in the episode "A Glitch is a Glitch." It is where Finn and Jake go to rescue everyone in Ooo, including themselves, before they are permanently deleted.
In the episode, Ice King showed to possess a vast knowledge of the source code, even managing to hack it to delete everything in the universe but himself and Princess Bubblegum, protecting their codes by freezing them with ice magic to prevent the Glitch from deleting them.
The universal source code is a cyberspace-like dimension that stores all the components of the entire universe inside hexagonal-shaped pieces called "source codes" that are linked to the material objects in the real world. Basically, it's a cyberspace with hexagonal pieces that are the basis to the structures of the universe linked by a grid. It appears as a vast empty space with millions of source codes in a kind of rainbow pattern color, and has glowing colorful lights and floating mathematical equations.