Synonyms: Security Certificate
SSL stands for Secure Socket Layer, and is used in connection with encrypted web traffic. Basically, it means that the data you enter and send is encrypted so that it cannot be intercepted and read between sender (you) and the recipient.
You probably know it from your browser, where you can most often see a small padlock next to the URL:
On websites that do not have a valid security certificate, you will most often be greeted by a warning in your browser, where you must decide for yourself whether you want to continue to the website, despite the potentially unsafe content.
However, it is also used for other things, such as when you e.g. connects to a server via the SSH or SFTP protocols.