Certain portions of crops in Israel are meant to be disposed of in certain ways: given to a kohein (priest), given to a Levite, given to the poor, and eaten by its owners in Jerusalem. There’s an order to when which gifts are to be taken and the Torah tells us not to deviate from the proper sequence.
After wheat is threshed, it is called tevel and may not yet be eaten. The first thing taken is the kohein’s portion, called terumah gedolah. (The Torah does not mandate a minimum size for this gift; rabbinically, it is one-fiftieth or 2% of the crop.) From what is left, we take maaser rishon, “the first tithe.” This is 10% given to a Levite. We take 10% of what is left. This is maaser sheini, “the second tithe,” which must be eaten by its owners in Jerusalem. (The second tithe is only taken in certain years; in other years we take maaser ani, “the tithe for the poor.”) First fruits (bikkurim), when called for, precede absolutely everything else.
The reason for this mitzvah is that delaying and doing things out of sequence increases the possibility of error. (“Delay” means, for example, not giving one’s terumah until after one’s maaser.) First of all, it’s a really bad idea to leave untithed produce lying around, as it might accidentally be eaten impermissibly. Aside from that, the tithes are a percentage of the whole, a percentage of the remainder, etc. Going out of order invariably leads to errors in one’s calculations. Tithes are important, so God impresses upon us to do them soon and to do them properly.
This mitzvah applies to both men and women but only in Israel. It is discussed in the Mishna in the third chapter of tractate of Terumah (which has no corresponding gemara in the Babylonian Talmud). In the Shulchan Aruch, it is codified in Yoreh Deah 331. This is #154 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos. This is #19 of the 26 mitzvos that can only be performed in Israel according to the list of Rav Yaakov Yisrael Kanievsky, the Steipler Gaon.