Well, you are connecting to a server on the undernet network, and for some reason that server is refusing you access.
the server might simply be busy, in which case all you need to do is wait, or try a different server. Undernet has lots of servers, and if you're having trouble connecting to a specific server, it would help if you'd tell us which server you're having trouble with.
In the mean time, you can try connecting using /server irc.undernet.org
That will connect you to a random server on the Undernet network, and if the connection isn't made or is refused, then it'll try another server.
If you get that on all of Undernet's servers, then try contacting them via their webpage to try to find out why your connection is being refused.
