Service Set IDentifier is basically the name of the wifi network. This is setup by the administrator of the network, and will be defined in the router or network server. Along with the SSID, there is also the passphrase which should never be the same as the login password. With an SSID, the person trying to connect can be confident that they are logging into the correct network which will be good if there are multiple wifi networks in the area such as in an apartment building.