SSL stands for Secure Sockets Layer. An SSL certificate is a data file installed on a web server which provides authentication and enables an encrypted connection.
An SSL certificate is needed to activate the padlock symbol in the browser and allow use of the https protocol. This reassures the user that the connection is secure and that personal information cannot be stolen.
Websites with an SSL certificate and which use the https protocol generally rank better in search engine results and have a better conversion rate due to enhanced customer trust.