A Kick is an action the owner of a world can do, for any reason or no reason, to temporarily kick out unwanted users in their world. Kicking a user involves typing the command "/kick [playername] [reason]". If [playername] is in the world when the owner types in the command and sends it to chat, that user will be temporarily banned from that specific world for five minutes. On the player's side, they appear at the loading screen after they've been kicked out, showing a box in the middle, saying,
"You were kicked by [Level Owner's Username]
[Level Owner's reason, if any]"
and then appear back at the lobby.
In the world, there is also a "*SYSTEM" message in the chat, which says "[levelowner] kicked [playername] - [reason if any]".
Should a level owner accidentally kick someone and they want to unban that particular player, right now, as of November 5, 2013, there is no way of unbanning that player, who has been kicked, without waiting the five minutes.
Level owners can kick themselves, although some do not believe so. Should the owner be kicked, the normal kicking message in the system will be replaced with "[levelowner] is no more".