i think the 1st thing basically means the server is full for normal users. It will still allow ircops, servers, services, special users etc to connect to it because they are in a different class.
It means the server isn't accepting any more connections of a certain type (class). Different classes might be determined for instance by those using a specific password, or those connecting from within the same country as the server, etc..
Since IRC uses case insensitive nicks, two people cannot have the same nick except with different case. CASEMAPPING tells the IRC client how the server determines what is upper and lower case (a case mapping). The rfc1459 token means that it uses a slightly adapted version of the case mapping specified in the IRC RFC 1459, which is that a-z is the lower case of A-Z, and that {}|~ are the lower case of []\^. There are also other common tokens, such as ascii (only a-z are the lower case of A-Z), and strict-rfc1459 which uses the exact specification from RFC 1459 (same as rfc1459 except for ~ not being lower case of ^).
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