A hidden wireless network is a wireless network that isn't broadcasting its network ID (SSID).
If you configure your wireless router to not broadcast the router Service Set Identifier or SSID, your network becomes a hidden wireless network. This prevents wireless devices from detecting the network, but it does not prevent the router from broadcasting the actual wireless network signal. Some use it as they think it makes them more secure when in reality it is security through obscurity. Your router might incorrectly use “closed” to mean hidden, and “broadcast” to mean not hidden.
Hiding the network name doesn't conceal the network from detection or secure it against unauthorized access. And because of the way that devices search for and connect to Wi-Fi networks, using a hidden network might expose information that can be used to identify you and the hidden networks you use, such as your home network. When connected to a hidden network, your device might show a privacy warning because of this privacy risk.
See blog post about "Why SSID Hiding Is Not Secure" for more information on this topic.