SSL in an acronym for Secure Socket Layer. SSL is primarily used to keep sensitive information safe via encryption while its being relayed online.
Lets say you have an email sign up form.
After a visitor enters their information and subscribe, that same information is relayed to your email newsletter provider online.
During this relay of information, if your website has SSL, only your website and the email newsletter provider can see your subscriber's information. However, if your website does not have SSL, the subscriber's information may be visible to the outside world.
If your website does not have SSL but you use a website widget that does, only information entered on that widget will be secured once relayed.
Serving your website via SSL is the best way to grow online.
Once you have SSL, you must take steps to ensure all elements on your website is served via SSL as well. If your website is secure but you have a widget on your website that is not secure, the web browser will warn the visitor that your website is not fully secure.