Windows uses backslashes for directories. Those backslashes have a special meaning in Dart strings. Hence, the r is used to make it a raw string, so that the backslashes won't take on their special meaning. Because I'm not on Windows, I couldn't run that code anyway, so I did a little trick so that I use "sample_file.txt" in the documentation, but the code does something slightly differently.

In any case, there's nothing to worry about here.