What is a network key and where can we find this in a laptop having no internet connection but in a wifi zone showing list of wifi networks

If you use WPA-PSK or WPA2-PSK encryption between a computer (with a wireless adapter) and a wireless router or wireless access point, it is possible to specify a "pre-shared key" (PSK) option. The pre-shared key is specified in the router or wireless access point by the router's administrator.

The owner of the wifi network would give you the network key if they allow you to join the network.