The Secure Sockets Layer is a system that guarantees a secure connection between your customers and your website. Any information your customers enter on your website will be encrypted, preventing it from being read or stolen by a third party.
SSL certificates are therefore essential for online shops and every website that requires customers to enter their payment details or other high-risk personal information. An added bonus is that Google favours websites secured by SSL. This will have a positive effect on your Google ranking and give you an advantage over your competitors without SSL.