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.
See blog post about "Why SSID Hiding Is Not Secure" for more information on this topic.