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The infield fly rule is a baseball rule that is intended to prevent infielders from intentionally dropping pop-ups in order to turn double plays (or triple plays). Without this rule, a defense could easily turn a pop-up into a double play when there are runners at first and second base. If the runners stay near their bases to tag up, the defense could let the ball drop, throw to third base and then to second, for a force-out at each base. If any of the runners stray too far from their bases, the defense could catch the pop-up, and double-off any runner that failed to tag up.
When the rule is invoked, the batter will be out (and all force plays removed) regardless of whether the ball is caught, thus negating the possibility for multiple outs. http://en.wikipedia.org/wiki/Infield_fly