enable mod's to shout in a voice room, where everyone else becomes muted temporarily while the mod says what he or she needs to say

So for a lot of IRC clients as a mod you can mute everyone temporarily in a "shout" function, where you can say what you need to say, if you might need a global announcement or something. This mutes everyone and allows the mod to be the only one talking, and have his/her voice come out on top.