Infield fly rule is a rule that stops the defense from intentionally dropping the ball and getting three outs instead of one.

The rule is simply this: if there are men on first and second base and the ball is popped up into an area which is deemed by the umpire to be in the infield, the umpire yells “infield fly”, this means the batter is out, for if having no this rule, the two runners on base would have two opportunities, to stay or to run. If they stayed on base, the fielder are able to intentionally not catch the ball, throw it to one base and then have it thrown to the other base, forcing both runners out. If they ran, the field could catch the ball and activate the double play, both runners tagged out on base. It is just not fair. So baseball invented the infield fly rule to make sure that a fly ball in the infield results in one fair out, not two unfair outs.