SSL stands for Secure Sockets Layer and establishes encrypted connections (https protocol) between the server of my site and your browser. The padlock symbol on the left site of your address bar shows you that the connection is safe.
After I bought the SSL option, I needed to adjust some links which I missed and deactivate a plugin that interfered with the https protocol. The developer tools of the browser where quite helpful to find the mixed content that still uses the unencrypted http protocol.
Using the https protocol is not only secure but can also have a positive effect on the search engine ranking (I’m curious about it).