How to protect AJAX call from facebook canvas app

Reading this example Ajax,

http://wiki.developers.facebook.com/index.php/FBJS/Examples/Ajax#Working_Example

I found the following line. I'm not sure what to understand from this, how do you "check sig values ​​for platform specification"?

"Note: for brevity, we trust $ _POST ['fb_sig_user'] without verifying the full signature. This is insecure as anyone can easily spoof a user action. Always be sure to use the Facebook object that comes with the client libraries or check the sig values ​​for the platform specification "