Metagaming is a technique of competition that is favored and approved of in real-world conflict and disfavored in competition. Essentially, it is the activity of using information and interactions outside the defined game arena or realm to win the match. This sort of thing is frowned upon in role-playing games and simulations, as it tends to disrupt the group suspension of disbelief that allows people to enjoy a fantasy or constructed world.

Example: four people (A,B,C and D) are playing a role-playing game, with A as gamemaster. B,C and D are in competition for a goal. B chooses to compete with C by examining their respective character or avatar attributes and choosing a strategy or tactic based on this information - say, that C's party is weak in warriors. This is normal. D, however, uses some piece of information about player C (as opposed to C's character) to formulate a strategy. In this example, let us suppose that D (a friend of C's IRL) knows that C never tends to have a spell ready for their magic user because they're disorganized and enjoys hand-to-hand combat modes more. D formulates a strategy based on surprising the magic user (in the game) because they know that the MU will be unprepared due to real-world traits of C.

This is metagaming, and is frowned upon.

Of course, in some competitions or conflicts (like, say, chess) then any means of advantage not expressly forbidden is eagerly sought, and this is nothing more than good psychological play.