A security certificate is a certificate granted by such companies like Verisign to Facebook, to say that their website is legitimate, not a fake and will provide you with the proper encryption/decryption codes.
It should be automatically sent to you by the website, all you have to do is accept it, if your computer does not already have it. It is also well known that certificates do not contact the authority to verify websites and that anyone can call up and buy a certificate under false ID and pretenses for hacking purposes.