As the name implies, it locks your Messenger from use. This feature is only available if you have MsgPlus! from http://www.msgplus.net installed. You will have to enter in a password to regain access to it. It has a very informative description right in the MsgPlus menu:
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Instant Lock give you the possibility to temporarily lock Messenger. It is mostly useful when you have to let someone use your computer for a short period of time and you don’t want to sign out.