SSID is the service set identifier which is a code attached to all packets on a wireless network to identify each packet as part of that network. The code is a case sensitive text string that consists of a maximum of 32 alphanumeric characters. All wireless devices attempting to communicate with each other must share the same SSID. Apart from identifying each packet, an SSID also serves to uniquely identify a group of wireless network devices used in a given "Service Set".
In other words, it is the nickname of your internet service which you can access by checking your ip address page or just simply calling your internet service.