SSL, or Secure Socket Layer, is a technology which allows web browsers and web servers to communicate over a secure connection. This means that the data being sent is encrypted by one side, transmitted, and then decrypted by the other side before processing. This is a two-way process, meaning that both the server and the browser encrypt all traffic before sending out data.
Another important aspect of the SSL protocol is authentication. This means that during your initial attempt to communicate with a web server over a secure connection, that server will present your web browser with a set of credentials, in the form of a certificate, as proof the site is who and what it claims to be. Therefore to implement SSL, a web server must have an associated certificate for each external interface (IP address) that accepts secure connections. The theory behind this design is that a server should provide some kind of reasonable assurance that its owner is who you think it is, particularly before receiving any sensitive information.