The Wireless Network trigger executes when a wireless network is connected to or disconnected from.
You can trigger on any wireless network connection, or only with wireless networks with matching names, or better yet BSSID (which is a more unique ID — lots of people have wireless networks named “Home” or “Work” for example).
If the name text field is selected when you connect or disconnect a wireless network, the name (or BSSID) of the network will be filled in for you (v7.0+).