There is a seven-year Sabbatical cycle, the seventh year of which is Shemittah, when the fields may not be worked. In four of the remaining years, ma'aser sheini, the second tithe, must be brought. (What happens in those last two years? Stay tuned for Mitzvah #474 and you'll find out!)
The second tithe was separated in the first, second, fourth and fifth years of the Shemittah cycle. After the first tithe was set aside for the Leviim, a second tenth of one's crop was separated in order to be brought to Jerusalem. If one lived so far from Jerusalem that carrying so much produce was impractical, one was permitted to redeem it. This entailed changing the produce for money, which was then brought to Jerusalem, where it was to be spent on food and drink. When this was done, one-fifth needed to be added to the value he spent in Jerusalem.
The reason for this mitzvah is the same as for the animal tithe in Mitzvah #360: it gives the people more reasons to go to Jerusalem, which is the spiritual center of the nation. Almost every family had someone in yeshiva in Jerusalem, who supported himself off of his family’s animal tithes and maser sheini.
This mitzvah only applies in Israel at a time when Jews are settled on the land and Jerusalem is inhabited by the Jewish people. It is the subject of the tractate of Mishna entitled Maaser Sheini. In the Talmud, it is discussed in tractate Rosh Hashana on pages 12a-b. It is codified in the Shulchan Aruch in Yoreh Deah 331. This mitzvah is #128 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos and #14 of the 26 mitzvos that can only be observed in Israel as enumerated by the Steipler Gaon.