Orlah, Chapter 2, Mishnah 1. This mishnah and those that follow deal with the question of how different categories of prohibitions and forbidden foods combine with one another with regard to nullification.
Terumah, Challah and Bikkurim: Nullified in One Hundred and One
The mishnah opens: "HaTerumah uTerumas maaser shel demai, hachallah vehabikkurim" - these are the items with which the first part of the mishnah deals:
"HaTerumah" - Terumah itself.
"uTerumas maaser shel demai" - one who buys produce from an am haaretz is required to separate terumas maaser. This law was stated regarding ordinary terumas maaser, and it applies to the terumas maaser of demai as well.
"Hachallah" - the challah separated from the dough.
"Vehabikkurim" - the first fruits.
All of these items share one status: they are all a form of Terumah given to the kohen. Therefore they "olin be'echad ume'ah" - they are nullified only in one part in a hundred.
The Measure of One in a Hundred: The Dispute Between Rabbi Yehoshua and Rabbi Eliezer
This measure of one in a hundred is in fact the subject of a dispute taught in the first chapter of Terumos, concerning what exactly "one in a hundred" means:
Rabbi Yehoshua: a se'ah of Terumah that falls into ninety-nine se'ah is nullified, since the total reaches one hundred, a hundred that includes the forbidden item itself. He did, however, require slightly more than a hundred, that is, that it fall into ninety-nine and a bit more.
Rabbi Eliezer: the measure of one in a hundred means one se'ah of Terumah falling into one hundred se'ah of permitted produce, together one hundred and one.
"Umitztarfin zeh im zeh"
Terumah, terumas maaser, challah and bikkurim combine with one another to make up the amount that requires nullification. If half a measure of Terumah and half a measure of challah fell in, they combine as though it were all Terumah or all challah, and the entire quantity must be nullified.
"Vetzarich leharim"
Even after the nullification, a person is obligated to lift out the quantity that fell in, whether of Terumah, of challah or of bikkurim, and give it to a kohen. The reason is that the monetary obligation remains: property belonging to the tribe of kohanim became mixed into the mixture. True, as far as the prohibition and the permissibility of eating are concerned, the matter has been nullified, but the monetary component stands in place: money belonging to the kohanim was mixed in, and therefore an equivalent amount must be lifted out and given to a kohen.
Orlah and Kilei Hakerem: Nullified in One in Two Hundred
The mishnah continues with orlah and kilei hakerem. These two prohibitions are prohibitions of benefit: any form of benefit from them is forbidden, not only eating. Terumah and everything similar to it are forbidden to be eaten by a non-kohen, but there is no prohibition of benefit; orlah and kilei hakerem, by contrast, may not be enjoyed at all. Since they carry an additional layer of prohibition, the Sages established that a second layer of nullification is required as well: instead of one in a hundred as with Terumah, "olin be'echad umasayim" - they are nullified only in one part in two hundred.
Furthermore, just as it was said of Terumah, challah and bikkurim "umitztarfin zeh im zeh", here too orlah and kilei hakerem combine with one another. However, "ve'ein tzarich leharim" - if they fell into a quantity sufficient for nullification, one in two hundred, there is no need to lift anything out and give it to anyone, since there is no monetary component here and the item does not belong to another party.
The View of Rabbi Shimon
Rabbi Shimon says: "Einam mitztarfin" - orlah and kilei hakerem do not combine with one another. This is his approach generally, that two different categories of prohibition do not combine. Terumah, challah and bikkurim are considered a single category of prohibition, whereas orlah and kilayim are two separate prohibitions, and therefore they cannot combine.
The View of Rabbi Eliezer
Rabbi Eliezer holds that they do combine with one another. To understand his position, a principle must first be set out: the measures stated here, one in a hundred for Terumah and one in two hundred for orlah and kilei hakerem, were stated only regarding a substance falling into its own kind, that is, when the forbidden item fell into the same type of food and there is no way to detect a difference in taste. But when a substance falls into a different kind, for example grain that fell into an entirely different type whose taste is noticeable, the nullification depends on taste: it is nullified only when it can no longer be tasted, and that measure is generally one in sixty, less than one in a hundred and less than one in two hundred.
Therefore Rabbi Eliezer says: "Mitztarfin benosen taam, aval lo le'esor":
"Mitztarfin benosen taam" - when the nullification depends on taste, they do combine. If half a measure of orlah and half a measure of kilayim fell into a mixture in which their taste can be detected, they combine.
"Aval lo le'esor" - they do not combine to create the prohibition in a case of a substance falling into its own kind, where the orlah and the kilayim fall into the same type of item and the required measure is one in two hundred. There they do not combine according to Rabbi Eliezer, and in this he agrees with Rabbi Shimon.
In summary: in this mishnah we learned that Terumah, terumas maaser of demai, challah and bikkurim share a single status: they are nullified in one in a hundred, they combine with one another, and one must lift out an equivalent amount because of the monetary component belonging to the kohanim. Orlah and kilei hakerem, which are prohibitions of benefit, are nullified in one in two hundred, they combine with one another, and there is no need to lift anything out. Rabbi Shimon disagrees and holds that two different prohibitions do not combine, while Rabbi Eliezer distinguishes between combining with regard to imparting taste, where they do combine, and combining to create a prohibition where a substance falls into its own kind, where they do not.