Voodoo is an addon which allows other players (when given permission) to send whisper commands to your character. The commands makes your character do something (if you can).
Some practical uses:
Allow other persons in your 5-man group to invite people (!voodoo invite <name>).
Let people put you on follow when you are afk and it doesn't matter much if they run into a wall (!voodoo follow <who>).
Someone is tired of you being slow roller and want to do it for you (!voodoo roll)
You are supposed to be replaced in a raid but are afk and kicking you won't port you. (!voodoo logout)
There are more commands to be used. Type "/voodoo whispers" in game for a full listing.
To disable unwanted persons from controlling your character, you first must allow them to do so. You can allow a specific person, everyone, your party/raid, or your guild. It is also possible to block a character even if they are in an allowed group. Allow people with "/voodoo allow <group>" or block them with "/voodoo block <character>".
Type "/voodoo commands" in game for a full listing of commands.