This message means the channel is set to +k (Key). This means that the channel is protected with a password that you need to know in order to join the channel. More often than not, ops (@) set a key to keep 'strangers' out of the channel. If you know the nickname of any of the channel ops then message one. Alternatively, try /topic #channel and see if the key's in their topic.
If you can't contact an op and it's not in the topic then you cannot enter the channel.