What are docstrings? How are they useful?

Q. What are doc-strings? How are they useful?

Answer =

A doc-string is just a regular Python triple-quoted string that is the first thing in a function body/a module/a class. When executing a function body (or a module/class), the doc-string doesn't do anything like a comments, but Python stores it as part of the function documentation. This documentation can later be displayed using the help() function.

So, even though doc-strings appear like comments (no execution), they are different from comments.