what does this mean, "robots.txt is forbidden?"

I just did an Alexa site audit. It found all kinds of nutty things on my website. But of all the nutty things it found, this one was the most puzzling. It came in the form of an error message that read:

Robots.txt Forbidden
The robots.txt file exists on your site, but access to it is forbidden. If you intended for crawlers to be able to read this file, you should change the file permissions to allow access to the file.

Does anyone here know what Alexa is talking about, and how I would change the file permissions as prescribed?

Thanks