Orlah, Chapter 2, Mishnah 3. Continuing the subject discussed in the previous mishnayos of this chapter - how one prohibited substance can help nullify another prohibited substance - the mishnah teaches three rulings:
"Ha'orlah ma'alah es hakil'ayim" - orlah that one has on hand can help nullify kil'ayim.
"Vehakil'ayim es ha'orlah" - and likewise the reverse: kil'ayim helps nullify orlah.
"Veha'orlah es ha'orlah" - orlah even helps nullify another type of orlah.
The Rambam's explanation of "veha'orlah es ha'orlah":
The Rambam understands that this is not speaking of two measures of actual orlah, since one type of prohibited substance does not nullify that very prohibition - it is simply more of the same thing. He therefore proposes that the case of "ha'orlah es ha'orlah" deals with one that is orlah and one that is neta reva'i, which is a sort of extension of orlah - the fourth year, which follows the three years of orlah. This is what the mishnah means.
"Keitzad" - working out the numbers:
"Se'ah orlah shenaflah lemasayim" - a se'ah, a measure, of orlah that fell into two hundred se'ah of chullin and was nullified at one part in two hundred, so that we now have two hundred and one. "Ve'achar kach naflah se'ah ve'od orlah, o se'ah ve'od shel kil'ei kerem" - afterward a se'ah and a bit more of orlah, or of kil'ei hakerem, fell in.
The original two hundred of chullin are not enough to nullify a se'ah and a bit more; they suffice to nullify only a single se'ah, and a se'ah and a bit more requires somewhat more than that. However, since the first orlah had already fallen in and been nullified earlier, we make use of the prohibited substance that was nullified, and we have two hundred and one - an amount sufficient to nullify the se'ah and a bit more at a ratio of one to two hundred. (The order can also be reversed: the kil'ayim fell in first, and the orlah fell in afterward.)
And this is how the mishnah concludes: "Zo hi she'orlah ma'alah es hakil'ayim" - that orlah helps nullify kil'ayim; "vehakil'ayim es ha'orlah" - and the other direction, that kil'ayim nullifies orlah; "veha'orlah es ha'orlah" - that orlah itself nullifies a measure of orlah. And as stated, according to the Rambam this is not orlah nullifying actual orlah, but rather orlah nullifying neta reva'i, or the reverse.
In summary: in this mishnah we learned that a prohibited substance that has been nullified joins with the chullin and helps nullify a prohibited substance that falls in afterward - orlah raises up kil'ayim, kil'ayim raises up orlah, and orlah raises up orlah. This is derived from the calculation in the mishnah: a se'ah of orlah that was nullified in two hundred of chullin leaves us with two hundred and one, which is enough to nullify a se'ah and a bit more that falls in afterward; and according to the Rambam, "orlah es ha'orlah" means orlah together with neta reva'i.