The address of a user is in the form of n!u@h.
The "n" symbol means "nickname", the "u" symbol means "user ID" and the "h" symbol means "host".
For example if a user's address is "RagingDemon!daredevil@vdp001.ath02.cas.hol.gr", then the nick is "RagingDemon", the user ID is "daredevil" and the host is "vdp001.ath02.cas.hol.gr". So, if you have selected ban mask number 2 which is "*!*@h", your ban will be placed on "*!*@vdp001.ath02.cas.hol.gr".
For more information about masks please refer to mIRC's help file or any other appropriate source.