Your innocent question is actually quite complicated. I’ll try to make this as concise as possible—so bear with me!

First the biblical source (Leviticus 19:23–25):

When you come to the Land and you plant any food tree, you shall surely block its fruit [from use]; it shall be blocked from you for three years, not to be eaten. And in the fourth year, all its fruit shall be holy, a praise to the L‑rd. And in the fifth year, you may eat its fruit.

We are forbidden to eat the fruits1 which grow upon a tree during the first three years after its planting.

These fruit are called orlah.

The fruit of the fourth year on a tree grown in Israel was brought to Jerusalem to be eaten there by its owners. The owner was permitted to do as he pleased with the fruit of the fifth year.

The prohibition of eating the fruit of the first three years applies no matter where the fruit was grown. There are however, a few halachic differences between the fruits of these years grown in Israel and those grown outside of Israel. Most notable among these differences is that fruit grown outside of Israel is forbidden to eat only if one is certain that it is orlah, whereas in Israel it is forbidden to eat a fruit whose orlah status is in doubt.2

Outside the Land of Israel, the laws of the fourth year apply only to the fruits of the vineyard.

The fruits of the fourth year can be eaten only when the Holy Temple is standing, by someone who is ritually pure. Unfortunately, today we do not have the Temple (a state of affairs we constantly plead with G‑d to change), and we are all considered ritually impure and do not have the means necessary for purification. Therefore, instead of bringing these fruits to Jerusalem, we “transfer” the holiness of these fruits to (a small amount of) money, and we discard that money.

The three years are not always complete, depending on when the tree was planted. This is where Tu B’Shevat plays a role; see Tu B’Shevat Q&A and Why Is Tu B’Shevat in the Winter? for details.

For more about orlah, click here.

All the best,

Rabbi Baruch S. Davidson