This week's Parsha states that when a person plants new trees in Eretz Yisrael, he may not eat the produce of those trees for the first three years, and in the fourth year he must treat the produce as holy (ויקרא י"ט:כ"ג-כ"ד). The Torah then says "And in the fifth year you may eat its fruit, that it may yield more richly to you its increase (of produce)" (שם פסוק כ"ה). This Posuk informs us of the reward which we receive for not eating the tree's fruits for the first three years and for redeeming the fruit of the fourth year in Yerushalayim, which the Halacha requires, as pointed out by Rashi (לפסוק כ"ד). The Posuk uses the phrase "להוסיף לכם תבואתו," "that it may yield more richly to you its increase," which apparently tells us that the purpose of not harvesting the fruits from these first four years normally is to raise an abundant crop in the fifth year.
Rashi offers a deeper explanation to this phrase, quoting Rabbi Akiva. He says that in this phrase, the Torah is responding to the Yetzer Hora, bad inclination, by one's saying that the fruit of the first four years will not be wasted by one's adhering to these Mitzvos, as the Yetzer Hora would suggest. Rather, one's harvest in the fifth year will compensate for the harvest of the first four years, and, on top of that, the normal harvest of that year will be larger. This is the reward promised by Hashem to those who keep these Mitzvos.
Rabbi Menachem M. Schneerson, the Lubavitcher Rebbe, offers an additional meaning to this phrase which goes beyond the apparent explanation, and is based on Kabbalah. He makes an analogy between these five years and the five "universes," or levels of spirituality. These levels exist in sequence from the least holy to the holiest. The first three years of harvest, during which the fruits are not edible, represent the three lower levels of spirituality: "Asiyah," "Yetzirah," and "Beriah," or, "Action," "Formation," and "Creation," which relate to man's physical actions in this world. At these levels, Hashem is revealed only partially because sinning is possible. It is at one of these levels of spirituality that most people function in this world. The fourth year of harvest, during which the fruits are to be taken to be eaten in Yerushalayim, represents the fourth level of holiness, "Atzilus," or "Emanation." At this level, everything is holy and connected to Hashem. The fifth level, "Kesser," or "Crown" is the highest and holiest of all five levels because Hashem is revealed most clearly at this level. This level corresponds to the fifth year of harvest which is most precious. Just as one has to go through the first four years of harvest in order to get to the most prolific and precious year, so too one should strive to work through the lower levels of spirituality and achieve success at the highest level, which, like the fifth year, will provide the greatest reward.