Usage: SERVER <server> [<port>]
This command is for connecting to a server with an IRC client. It will disconnect you from the server you are using and try to connect you to the server you specify.
/server 6667
With most clients if no port is listed it defaults to 6667, the standard IRC port. If the server you were using goes down or if you are experiencing significant lag (delays between you typing things and people seeing them) then you might want to switch servers. The server command will cause the client to try to connect to whatever server you specify.
Note: this is a client-side command, some clients may not support it or have slightly different syntax.