This prompt means that your wireless network is already configured with some form of security, and now you need to provide the matching key for it.

If the person who configured this network is available, you should ask to be provided the network key.

If you are the owner of the network, you should enter the configuration pages of your AP or router. Depending on the type of device and the security used, you may or may not be able to see the key. If you are not able to see it, then you can change the key to something you can remember. Be aware that such a change would affect everyone using the wireless network.

If you have a wireless router, these settings are usually found on the configuration pages of the router. Otherwise, your AP will have different pages. Your user manual will give you the steps to perform. If you do not have the manual any longer, visit the support web page for your device. There you often can find links to download the manual.