Your router has encryption enabled. To use it, the client needs to enter in the security key that has been entered on the router. This key is most-likely not written on the outside of the router. The person who enabled encryption on the router would have entered in a key, although some routers might have encryption enabled with a default key but this is uncommon.
If you were not the person who enabled encryption on the router and do not know the key, you will need to log into the router using your computer and look around in the settings to find the key. Note that most routers have a special username/password to log into their web interface... this is entirely separate than the encryption key you're searching for, so you might have to dig this up as well. Check the manual (or manufacturer's website) for the default username/password for your router. If the default doesn't work, someone else changed it and you might be faced with having to do a hard reset of the router in order to clear all its settings and set it all up again from scratch.
Assuming you managed to log in, the encryption key is a long string of hex digits (0-9, A-F) so make sure you enter it carefully into your Wii.