The following video will explain the infield fly rule in depth. But first, let's keep it simple with some basic principles for both baserunners and fielders.
If you hear the umpire announce "Infield fly!" or "Infield fly if fair!", here is what you need to do:
Catch the baseball
If a baserunner does not tag up, throw the the base that they left early to double or triple up the baserunners.
If you accidently drop the baseball no worries the batter is automatically out.
If a baserunner attempt to move to the next base, get the out by tagging the runners. It is not a force play, you must tag them.
Stay on the base!
Stay On The Base! Even if the ball is dropped you do not have to run.
Only attempt to take the next base if the fielders commit an error allowing you to reach the next base safely.