This command will mass kill users from the same IP address, hostmask or channel. It is normally used when a large quantity of users from the same address or channel need to be removed from the server and in the case of a channel where you want to clear a channel out completely.
Example:
/mkill pp23.ntsvr4-2.tor.idirect.com cloning
This is especially useful if the users are changing their nicks because it allows you to kill by address. It is a good idea to check exactly who will be killed by this command before you send it by using the /who command.