It's the pattern behind how you think, react, feel and what you expect. It was built over time, shaped by what you've experienced.
It drives what you believe, what you tolerate and what you expect from yourself and life. Some of it works, but some of it keeps you in the same cycles.
Most people try to change what's happening on the surface. (If it were that simple, you wouldn't be ending up in the same patterns.)
So what do you do?
You Stop trying to fix the surface, and you go to to the source.
The Source Code, is what's running underneath your thoughts, reactions and decisions.