This refers to premium software that has had its license verification or "phone home" features removed. It allows the software to run without a valid purchase key.

The Hidden Risks of Using Nulled and Patched Android App Source Code

Many forums advertise "patched" source code as being "cleaned of trackers." In reality, the person patching the code is often replacing the original developer's trackers with their own. You aren't getting a cleaner version; you're simply switching who is monitoring your data. Better Alternatives to Nulled Code