A programmer's joke meaning you should read the source code. It is also a Star Wars themed pun on Obi-Wan Kenobi's "Use the Force, Luke!"
A short form of this is UTSL.
"Most people who come here for help do know how to read the source code. In case they can't, I tell them what to do. If they can, though, what I usually say is 'Use the Source, Luke!'."
by cutesy pastel living doll March 1, 2013