This is the name of a concept to combine source code and highly structured human-readable documentation, such that the main artefact is the documentation, and the secondary artefact the full source code extracted from it. Any change to the source means changing the explanatory documentation of the goals and reasoning at the same time. It was invented by no other than Don Knuth, and he, together with his collaborators, wrote the TeX program in it, to show how it is useful in practice.

I myself have used the technique in various occasions at work, for example when taking on an important but almost unreadable piece of code I inherited at work, to preparing sources for a complex algorithm for handover when I was leaving another larger project, and also in leisure coding.

The tool which is most widely used for this approach today is Emacs org-mode. It has the advantage that it works for every programming language, and you will likely find full examples for all languages you use, but many other tools (for example for vim) exist.