SSID stands for Service Set Identifier. This is the name given to a wireless network. When connecting to a wireless network, the user must know this name along with the password, or wireless key, if the network is secure.
Typically, the SSID is broadcast so that it can be seen if a device searches for available wireless networks. Most routers have a default SSID name, but these can be changed by the wireless network administrator to any name or phrase, provided that it's less than 32 characters. SSIDs can be hidden so that only users who are aware of the network and have the proper SSID and key can access the network. Even in these instances, the user can still save the SSID for automatic connection to the network.