An SSID (Service Set Identifier) is the public name of a wireless local area network (WLAN), which serves to differentiate it from other wireless networks in the area. For Google Fiber, the SSID is the network name you specify when you configure your Wi-Fi network. Any wireless devices that connect to your network must use this SSID.
By default, your Network Box broadcasts a beacon signal, announcing its presence by providing the SSID. Broadcasting the SSID displays the name of your network in the list of available networks when nearby users try to connect their wireless devices. If you don't want arbitrary wireless devices attempting to connect to your network, you can disable broadcasting of your SSID.
If you assign a different network name/SSID to your 5 GHz wireless network, the SSIDs for both 2.4 GHz and 5 GHz are broadcast by default, and you must enable/disable broadcasting for each of them separately.