IRC hostmask cloak replaces the display of your IP address or host name with a cloak
an example would be
libra-away is connecting from email@example.com <is how an uncloaked host would look
when it is cloaked it would look like this > libra-away is happy@2B0381.CA26FF.1EBC87.347CDB <
Hence hiding your real ip from anyone who does a /whois on you. Most networks have cloaking automatically as you join it but some you need to request a hostmask.