When you visit an unsecured website, your browser makes a connection with that site and transmits information in the public. Filling out a form, or entering your credit card on an unsecured website means that hackers could intercept the information and thus steal your data.
By comparison, a secure website with SSL enabled will establish a tunnel between the server that hosts the website and your browser. Any hackers that try to intercept information fundamentally cannot access the tunnel -- and thus any information you share on the site is secure. The secure connection and authentication