The network security key.is the "password" that is required for you laptop to connect to the wifi router.
(sometimes called a network key, a passphrase, or a network password)
You'll need this to connect.
Yesterday when you connected you would of enetred in this password into your laptop.
If you dont know it then someone else that connects might.
When you get this password and you go to enter it again there should be an option that allows you to "connect automatically" - it normally is a check box that just needs checking (ticking) before you connect, that way the laptop knows to connect automatically without requiring the password
Sometimes the router/modem comes with a password or network security key printed underneath on the base. Its the default one that comes with the unit BUT normally when a router/modem is set up the password is changed to something else by the user.