SSL stands for Secure Sockets Layer.
Secure Server Layer provides a secure connection between a computer and a web server. It does this by encrypting the data being transmitted so that only the server and the end user's machine can understand the data that is being transmitted. The purpose of this is to prevent unwanted hackers stealing any data during transmission.
You'll notice any website with SSL installed will display https in the address bar; those without will display http.
SSL was the most widely used security protocol on the Internet and was designed to provide encrypted transmission of data. However it was later succedded by the more common form of security TLS (Transport Layer Security).