Hello thebeartrap!

It happens when a server doesn't allow to execute some command to get info about some settings. You need to click the link "Server audit" and under "Web Server" section you will find more details. For example, the installed mod_rewrite can be found by this command under SSH 

apachectl -M 2>&1 | grep rewrite_module

So if your site doesn't have the troubles in work you may pass this "FAIL" about WebServer.