SSL is an abbreviation used for Secure Sockets Layers, which are encryption protocols used on the internet to secure information exchange and provide certificate information.
These certificates provide an assurance to the user about the identity of the website they are communicating with. SSL may also be called TLS or Transport Layer Security protocol.
In most modern web browsers users can click on the SSL icon displayed in the address bar to view certification, identification and other information about a website.
SSL and TLS protocols are not just limited to websites. They are also used in email, SFTP, and various other internet technologies.
WordPress sites can also benefit from SSL by adding an SSL certificate to their web hosting plan. There are also WordPress plugins available which allow users to setup their SSL certificate information through out their WordPress sites.
Most websites use SSL on their payment gateways and eCommerce sites.
As a user, it is always important that you never enter your payment / credit card information on pages that are not secure.
Modern browsers like Firefox and Chrome will show errors and sometimes block rendering of improperly secured pages on a site.