An SSL Certificate is required on a website that shows or asks for sensitive data such as social security numbers, credit card numbers, or other sensitive information. Through the use of this certificate that is installed on the web server, it encrypts this sensitive data. When you see an open or closed padlock on your browser it is letting you know if the web page you are visiting is secured with an SSL Certificate. Another indication is whether it says HTTP: (unsecured) or HTTPS: (secured) in the website URL. It is a good practice to glance up there looking for the padlock or HTTPS before entering your credit card number online.