In IT slang this is told when there is none or poor documentation available for a given program, thus the only way to understand its behaviour is to read through the source of it. This is only possible if one has access to the source code itself, which is not case with most of the proprietary software, but is with open source and free software.
My name is Alex Beregszászi. You can email me on this domain, the address is my first name at this domain. You can also find me online on github, linkedin, twitter and medium.