Secure Sockets Layer (SSL) is a security mechanism that is used to encrypt communications between two devices. One of the parts are often a web server (eg online shop) and the other is a web browser (eg Firefox or Chrome). The idea of encrypting the data sent to and from these two devices is that no one else on the network (usually the Internet) should to be able to eavesdrop or manipulate information. If you buy something on the internet, it is natural to use SSL to encrypt credit/debit card information for example.

Source: http://sv.wikipedia.org/wiki/Secure_Sockets_Layer