A string that appears as the lexically first expression in a module, class definition or function/method definition is assigned as the __doc__ attribute of the object where it is available to documentation tools or the help() builtin function.

self-documentation for functions and variables

In programming, a docstring is a comment included in source code to document the purpose of a piece of code. Unlike conventional source code comments, or even specifically formatted comments like Javadoc documentation, docstrings are not stripped from the source tree when it is parsed, but are retained throughout the runtime of the program. This allows the program to inspect these comments at run time, for instance as an interactive help system, or as metadata.