WPS stands for Wi-Fi Protected Setup. This is a process that let you add a wireless client to the network easily, and you don't have to specially configure the wireless settings like SSID, security mode, and passphrase.
You can select to add a wireless client either through the "Push Button" or "PIN Number".
With the "Push Button" method, you have to complete the process by clicking a button on the client (either a hardware button or a software button on the client's utility GUI).
With the "PIN Number" method, you have to enter the client's PIN here. You have to start the client WPS process at the same time, and you can find the client's PIN on the client's utility GUI.