The Torah in Parashat Re’eh (Debarim 14:22) commands tithing one’s produce and bringing the tithe to Jerusalem, where he must then eat it. This tithe, called Ma’aser Sheni (“the second tithe”), is taken on the first, second, fourth and fifth years of the seven-year Shemita cycle. (The mnemonic representing these years is “Abeda” (literally, “lost object”), which is spelled “Alef,” “Bet,” “Dalet, “Heh,” alluding to the first, second, fourth and fifth years.) We refer to this tithe by the name “Ma’aser Sheni” because it is taken after the initial tithe, which is given to the Leviyim. Unlike the first tithe, Ma’aser Sheni is not given away, but is rather brought to Jerusalem and eaten there by the owner. If the owner finds it too difficult to bring all this produce to Jerusalem, then he has the option of transferring the produce’s sanctity onto money. He then brings the money to Jerusalem and uses it there to purchase food, which he eats in the city. When he transfers the sanctity of the Ma’aser Sheni onto money, he must add a Homesh (one-fifth) to the value of the produce.
The reason for this Misva, as the Sefer Ha’hinuch explained at length in an earlier context, is that the Torah wants people to spend time in the city of Jerusalem, which was the center of Torah scholarship. In order to connect people to Jerusalem, and thereby connect them with righteous, learned scholars, the Torah introduced several obligations that require a person to go to Jerusalem. As a result, people are likely to purchase homes in Jerusalem, or to send children or relatives to live there, so they have a place to stay. This has the effect of strengthening a person’s connection to the holy city of Yerushalayim, and, by extension, to the Torah scholars who live there.
The Misva of Ma’aser Sheni requires taking a tenth of that year’s produce; one cannot set aside produce as Ma’aser Sheni for a different year’s produce. The “fiscal year” with respect to Ma’aser Sheni as far as fruit trees are concerned is Tu Bi’Shbat. If a tree reached a certain point of development by Tu Bi’Shbat of the second year of the Shemita cycle, then one-tenth of its produce is designated as Ma’aser Sheni. If, however, the tree reached this point after Tu Bi’Shbat, then the fruit is considered the produce of the third year of the Shemita cycle. The produce of the third and sixth years is subject to a different tithe – Ma’aser Ani, which is given to the poor, instead of Ma’aser Sheni.
The Torah obligation of Ma’aser applies only to Dagan (the five principal grains), Tirosh (wine) and Yis’har (oil). Other produce requires tithing Mi’de’rabbanan (by force of Rabbinic enactment).
Furthermore, the Torah obligation applies only when the majority of the Jewish Nation resides in the Land of Israel. Nowadays, tithing is required for Israeli produce Mi’de’rabbanan. A person who has produce separates Ma’aser Sheni, and then, since we are all Teme’im (impure) and thus forbidden from eating Ma’aser Sheni, he transfers the sanctity onto a special coin. After transferring several times, the coin is either thrown into the ocean or destroyed. Due to the special laws that apply to Israeli produce, one who purchases produce in Israel, or Israeli produce outside Israel, must ensure that it is under supervision by a reliable Kashrut agency.