Can someone explain the infield fly rule (very simply, please!) and what you're supposed to do if you're on first base and someone hits a fly ball?

It only applies when there is less than 2 outs and there are runners at least on first and second base. If there are runners only at first or first and third, the rule does not apply. When a pop fly is hit, depending on how close it is hit to you, go about your body length and a half away from the base.

I don't know the whole thing but it's when there a so many outs and like a certain amount of people on base or something like tht. But if the batter hits a pop-up in the infield he/shes automatically out.

you run forward after you kick off the base go half way and make your mark and watch to see if they catch the ball if so run back to 1st as fast as you can if not then hustle to 2nd!and just go back if your coach yells back you dont wanna get ye'lll at!!

on an infield fly you stay on first base because you are able to run at your own risk and there is no force out because the batter is out.

For an infield fly rule to be “In Effect”:
1) There has to be less then two outs.
2) Runners have to be on 2nd and 1st base or 3rd, 2nd and 1st.
3) Ball has to be fair. (You will hear the umpire state “Infield fly if fair”.) If dropped in or touch in foul territory it is a foul ball. (The balls location, not the player, determines fair or foul.)
4) Infielder has to be able to catch the ball with reasonable effort. (An outfielder may catch the ball but has to be able to have been handled by an infielder with “reasonable effort”.) Protection of the runners is the intent.
5) The rule is understood if the umpire calls infield fly or not.
6) Can not be enforced on a bunt

What takes place: when applied, Batter Runner is OUT. (If the ball is caught or not)
Runners are no longer forced to run. They advance at their own risk.
NOTE: If ball is dropped or not caught, runners do not have to tag up.
If ball is caught runners have to tag up at first touch to advance, and are subject to be appealed if they left base too early.
Or they may choose not to move at all, the safer move.

Some misconceptions: It is NEVER a dead ball…Live and any action can take place, only the batter is OUT instantly. And if they continue to run may be subjected to interference.
If runner is just on first or third / or first & third. Rule is NOT in effect
If there are 2 outs: Rule is NOT in effect
A line drive is not an infield fly. (Other rules apply to an intentionally dropped ball on a line drive)

The Rule is to protect the offensive runners from being forced out and or multiple outs on a routine catch.

Vince Cordi
NHFS umpire: 5 years
ASA umpire: 7 years
National Championship Umpire: 3

If there was no infield fly rule, the following could happen: Runners are on first and second with less than 2 outs. Pop fly is hit to the third baseman. He intentionally drops the fly ball, picks it up, touches third and then throws to second for a double play. It's an easy double play because both runners are tagging up on their bases expecting the ball to be caught.

Infield Fly Rule:

http://www.qcbaseball.com/baseball_rules/infield_fly_rule1.aspx

- - - - [Here is an EXCERPT from the website for you] - - - -

Infield Fly Rule

The infield fly rule always seems to be a point of confusion for players, coaches, and parents watching a game. The purpose of the infield fly rule is to prevent a defensive team from purposely dropping or not catching an infield fly with the intention of trying to turn a double play.
Why we need the infield fly rule

If there was no infield fly rule, the following could happen: Runners are on first and second with less than 2 outs. Pop fly is hit to the third baseman. He intentionally drops the fly ball, picks it up, touches third and then throws to second for a double play. It's an easy double play because both runners are tagging up on their bases expecting the ball to be caught.
When can the infield fly rule be called?

* Less than 2 outs. It's purpose is to prevent a double play.
* Runner on first and second, or bases loaded. (There must to be a force at third base)

So where's the confusion?

As usual, the confusion is in the details and the combination of things that can happen. One point of confusion is that many people mistakenly think that the infield fly rule applies when there is only a runner on first base. This isn't the case. When there is no force play at home or third, the only way to turn a double play, by intentionally dropping the ball, is if the batter doesn't run to first base. In this case the infield fly rule doesn't apply and the defensive team could turn a double play. I don't think anyone will feel bad for the batter in this scenario.

So now that we know the game situation that the infield fly rule can be called, let's move on to the call itself. The main thing to remember is that the infield fly rule is a judgment call by the umpire. If the umpire determines that a player can make the catch with ordinary effort, then he/she can apply the rule. After the ball is hit in the air, you should hear the umpire yell, "Infield fly, batter is out." If you don't hear the umpire yell that, then assume the rule doesn't apply. The rule is written to give the umpire the ability to determine whether to apply the rule or not. Here is an example when the rule may not applied by the umpire. Runners on first and second. The third baseman charges toward home on a bunt coverage. The batter swings away and pops the ball up by third base. The umpire determines that the ball cannot be caught with ordinary effort, so he says nothing. The infield fly rule is not applied and the third baseman can pick the ball off the ground (if he missed it) and touch third base for a force out if the runner hasn't made it to third yet. Make sure you and your players don't assume that the infield fly rule is automatically enforced based on the situation. It is still a judgment call by the umpire.