That would be a password that is stored in your router. It is required to allow a wireless device to connect to your wireless router for internet access.
Your network security key is a special password that you generate using Wpa when you set up your home network. The wireless security key prevents anyone who comes within range of your wireless router from connecting to it unless you give them the password. Naturaly you will enter that pass key or security key into any of your wireless devices so that they can connect to your network. Your router also has a guest pass key that you can let your friends use when they come over and that allows them access without having to give them your network security key. The guest pass key is located inside the routers settings.