The original game
Rocksmith is a game put out by Ubisoft in 2011 and upgraded twice since which allows a player to connect an electric guitar or bass to a computer (Windows or Mac) or console (PS3/4/5, XBox) for the purpose of learning to play lead, rhythm, or bass tracks of given songs. In its unmodified form, Rocksmith requires the use of a so-called ‘Real-tone cable’) to provide connectivity between game and machine.
Players aim to play the notes shown to them on the screen and are scored according to how well they play tracks. For the purpose of practice, sections of a track can be selected and looped, and speed and difficulty can also be varied.
The game came with a number of songs pre-packaged with the option of purchasing additional content on a per-song or per-song-pack basis; these are referred to as ‘ODLC’, or ‘original downloadable content.
See the Wikipedia article on the subject for more details.
User extensions
Since then, a user community has come into being under the name ‘CustomsForge’ which provides support and additional software tools for the original game. Some members have undertaken to create custom downloadable content for the game (CDLC).
Having learned the ropes and produced one CDLC myself, I’m happy to offer my insights on the subject, for whatever they are worth, at Creating a Rocksmith CDLC.