It means that the Fielder can't intentionally drop the ball to try to get a double or triple play (it only applies when runners are on base)
So say you team has a runner on 1st and second and no outs and the batter pop out within the infield the umpire will usually call the infield fly rule so no matter what happens the runner is out even if the fielder drops the ball!
An INFIELD FLY is a fair fly ball (not including a line drive nor an attempted bunt) which can be caught by an infielder with ordinary effort, when first and second, or first, second and third bases are occupied, before two are out. The pitcher, catcher and any outfielder who stations himself in the infield on the play shall be considered infielders for the purpose of this rule.
When it seems apparent that a batted ball will be an Infield Fly, the umpire shall immediately declare “Infield Fly” for the benefit of the runners. If the ball is near the baselines, the umpire shall declare “Infield Fly, if Fair.”
The ball is alive and runners may advance at the risk of the ball being caught, or retouch and advance after the ball is touched, the same as on any fly ball. If the hit becomes a foul ball, it is treated the same as any foul.
If a declared Infield Fly is allowed to fall untouched to the ground, and bounces foul before passing first or third base, it is a foul ball. If a declared Infield Fly falls untouched to the ground outside the baseline, and bounces fair before passing first or third base, it is an Infield Fly.
Rule 2.00 (Infield Fly) Comment: On the infield fly rule the umpire is to rule whether the ball could ordinarily have been handled by an infielder—not by some arbitrary limitation such as the grass, or the base lines. The umpire must rule also that a ball is an infield fly, even if handled by an outfielder, if, in the umpire’s judgment, the ball could have been as easily handled by an infielder. The infield fly is in no sense to be considered an appeal play. The umpire’s judgment must govern, and the decision should be made immediately.
When an infield fly rule is called, runners may advance at their own risk. If on an infield fly rule, the infielder intentionally drops a fair ball, the ball remains in play despite the provisions of Rule 6.05 (L). The infield fly rule takes precedence.
The infield fly rule is very interesting. My best friend is an umpire and so that is how i know that this is it:
If there are runners on 1st and 2nd with less that 2 outs and the ball is popped up in the infield (so pretty much anywhere surrounded by the dirt and in). This does NOT include foul balls.
They do itt because if there is a ball popped up lets say on the pitchers mound, and there is runners on 1st and 2nd with 0 outs, and they pop it up and the defenders purposely miss it and then only have to throw it to 3rd and 2nd for an easy double play. so therefore they call it an automatic out so the runners don't have to run half way to the base in case they miss it.
I there's a man on base and the batter pops it up in the infield, it is an automatic out because.....
If a fielder were to catch the ball, it would be 1 out, but if he intentionallly missed it, it would force the other base runner to run, and they could get 2 outs with a double play.
If there is at least one runner on base and a pop fly is hit that would not leave the infield, it is an automatic out. (An out is recorded even if the fielder drops the ball). This is prevent unfair double plays. For example, imagine if the bases are loaded. If there is a pop fly, the runners will hold because they assume the infielder is going to catch it. Well if he dropped that ball, he could easily throw out 2, or even 3 runners.