I think people are one factor. Source does, when done well, communicate intent and implementation precisely to other people. If the language has enough semantic information it can communicate some part of the specification as well. Knowing what to write and ensuring your software does what you think it does is an important part of a project. It takes working with people to build the correct understanding of the specification and requirements.

But to a degree it is also about the code. Sometimes your requirements are: be fast enough. In which case you need to think about code: instructions and data.