"WPA key" refers to the Wi-Fi Protected Access (WPA) password you need to connect your PS3 to your wireless network. It's essentially the key that unlocks access to your router and allows your PS3 to join the network.
Here's what you need to know:
To connect your PS3 to a Wi-Fi network:
Important Note: If you're having trouble connecting, make sure you are entering the correct WPA key. It is case-sensitive.