I believe it means responding to a request to "www.example.com" by sending a 301 redirect to the corresponding URL at "example.com".
Recently, I've been discovering that the www is a good idea in certain ways. Cookies are sent to servers based on the domain name, so having that www there is a good way to make sure that you'll be able to have a subdomain which isn't sent those cookies (say, because it's served in a way you don't fully trust, or because it's a static file subdomain which shouldn't have to wait for the browser to send it superfluous cookies). I'm kinda not sure how it all works right now, but it's got me paranoid. But I digress; whether the redirection should be done is not the question here. ^_^