In Python, a docstring is a string placed at the start of a function, class or module. It describes the public behaviour someone needs to know, such as what a function expects and returns. A comment inside the function might explain an implementation choice; the docstring speaks to someone using the function without reading every line.
Good documentation can be short. State the contract accurately, especially any assumptions that are easy to miss. If the behaviour changes, the docstring should change with it.