This command will voice (+v) you in the room. You must be a VOP or higher to use this command in the selected room. Much like the op command. You must be an AOP or higher to voice someone else.
/msg chanserv voice #spiderslair Jenkins
This command is mostly useful for voicing yourself in rooms where you are a VOP. When voicing others you would generally just mode them +v, but you can use services if you wish. Being able to voice yourself with services means that you do not need to leave and rejoin the room to be revoiced.