Usage: VOICE <#channel> [<nick>]
This will voice a user or yourself in a channel, but the mode change is done by the server, you need to have AOP or higher access to the channel to use this command on other users and VOP access to use it for yourself.
It's particularly useful if you are not opped or voiced but need to voice someone else or yourself.
/voice #mychannel Zorgor
See also CMODE +v and OP.
This replaces the service command /CS VOICE.