that means it cannot find or resolve the IP for the address you want to connect to. One of the reasons for that could be that the address simply does not exist (check spelling) or that the server you are trying to connect to is down for some reason. (Or that you are not connected to the internet).
If the spelling of the address is correct, the server is probably down (or DNS is down), try another server of that network. Most networks have several servers and most of the time you can find other server addresses on the IRC network's website.