A term usually used as a put down to those who ask obvious questions and have their questions answered in a sarcastic way. It resembles the classic burned or owned, but more specific to question-asking. In forums "teach!" is often followed by an emoticon wearing a graduation cap and holding a pointer.
n00b: Why can't I post in the rest of the forums?
member 1: Surely you already know that after having read the rules before joining.