Source Code refers to the uncompiled text programs that make up HeroEngine or other 3rd party tools we provide. Source code is usually written in C++ or C#, but may be any any programming language. Source code should not be confused with Script code, which is always written in HSL.
Of the various license types, only "Full" and "Source" code licenses have access to any of the source code. A source code licensee has access to the complete codebase. A "Full" licensee has access to only part of the source code, primarily to the client source, to allow the creation of a customer-specific front end for your game.