Over the last few months we have made a big push to secure the majority of our clients' sites with an 'SSL Certificate'. If we haven't got to yours yet then we will have done by the end of next week.
The reason for this effort is that it shows all visitors to your site that you are secure and that they can communicate and interact with you in confidence. The SSL certificate means that communication with your website is now encrypted, reducing the opportunites for fraudsters to pinch your essential details from the ether.
Visually, your web address will now appear as 'https://' and there will be a small green lock in the browser address bar.
New Metric for Search Engine Optimisation
Google have indicated that it will use a site's secure status as a 'minor' ranking signal and that they wil start to index secure pages ahead of unsecured pages.
Chrome and Firefox are Pushing the move to a more secure web
Both Chrome and Firefox are already starting to indicate that 'HTTP' sites are insecure and depending on your security settings they will not display 'insecure' content at all.
Seeing as you asked ...
The way that an SSL certificate works is described in the diagram below.