The Torah in Parashat Korah (18:26) commands that a Levi, who receives the Ma’aser Rishon tithe from farmers’ agricultural produce, is required to separate ten percent from the produce he receives, and to give it to a Kohen. This donation is called by the Torah “Ma’aser Min Ha’ma’aser” – “a tithe from the tithe,” and the Sages refer to it as “Terumat Ma’aser.”
The Torah commands that the Levi donate to the Kohen “Mi’kol Helbo,” which means that he must give the choicest portion of the produce he receives to the Kohen as his Terumat Ma’aser.
The Sefer Ha’hinuch explains that the Torah required the Leviyim to give a portion to the Kohen to remind them of the Kohanim’s special stature. Although the Leviyim themselves were appointed a special role to serve in the Bet Ha’mikdash, they must recognize the unique prominence of the Kohanim, who perform the service of offering sacrifices in the Bet Ha’mikdash. Moreover, the Sefer Ha’hinuch writes, if the Leviyim would not be required to donate a portion of the Ma’aser Rishon that they receive, Beneh Yisrael might look down on them, and say, “You received only grain; but we receive credit for a Misva.” G-d wanted the Leviyim to also have a Misva to fulfill with their produce, just as Beneh Yisrael have Misvot to perform with theirs, and so He commanded the Leviyim to donate a portion of the produce they receive to a Kohen.
If a Levi took his Ma’aser Rishon from produce that was still in the form of stalks, before the stalks were processed into grain, then he is not permitted to give Terumat Ha’ma’aser before processing it himself. Rather than give a Kohen one-tenth of the stalks, he must process the produce and then donate one-tenth of the grain to a Kohen. In such a case, he gives only one-tenth of the actual grain, and not one-tenth from the straw that is separated over the course of processing. If, however, he separated one-tenth for Terumat Ma’aser before processing the produce, then after processing the stalks and producing grain he must give the Kohen the entire portion, even the straw, since it had all been designated as Terumat Ma’aser.
If a Levi’s Terumat Ma’aser from a beverage he receives amounts to less than 1/64th of a Log, then he does not have to go through the trouble of giving it to a Kohen, and instead burns it. The exception to this rule is Terumat Ma’aser of wine and oil, which must be given to a Kohen even in very small amounts, unless the liquid had become Tameh (impure), in which case the Levi can burn it instead of going through the trouble of finding a Kohen.
This Misva, of course, applies only to Leviyim, and only when the laws of Terumot and Ma’aserot apply.