Orlah, Chapter 1, Mishnah 6. This mishnah rides on the tail of the previous one, which closed with the idea of nullification - how orlah becomes nullified within a permitted fruit. Our mishnah also deals with nullification, but in a different context: the law of "ein mevatlin issur lechatchilah" - one may not nullify a prohibition deliberately. A person may not create a nullification; if the nullification happens on its own, then it happens, but one may not cause it to happen.
The ruling of the mishnah:
A plant that is forbidden because of orlah, or a plant that is forbidden because of kilei hakerem (having been planted within a vineyard) - both of these prohibitions are nullified only in one part in two hundred and one. When such plants, whether of orlah or of kilei hakerem, have become mixed in among other, permitted plants that carry no halachic problem at all, one may not pick them.
The reason for the prohibition:
As long as the plants are still attached to the ground, they are not nullified. Nullification applies only to things that are not attached to the ground, so it turns out that whoever detaches them from the ground is the one actually causing the nullification. Therefore one may not pick them, since by that very picking he would be nullifying them in one part in two hundred and one.
After the fact:
If he nevertheless did pick and detach them, they are indeed nullified in one part in two hundred and one - provided that he did not intend to pick those particular ones. The nullification comes about on its own, since one may not nullify a prohibition deliberately.
The view of Rabbi Yosi:
Rabbi Yosi disagrees and permits this case. Even though as a rule one may not nullify a prohibition deliberately and may not bring about a nullification by hand, here it is permitted, and the plants are nullified in one part in two hundred and one.
Rabbi Yosi's reasoning, as the Gemara in Gittin explains: a person does not want to render his entire vineyard forbidden on account of one single thing that was planted in it. The implication is that we are dealing with a very rare case, and in such a rare case the Sages did not extend their decree of "one may not nullify a prohibition deliberately," since it is considered an uncommon occurrence.
In summary: In this mishnah we learned the law of a plant of orlah or of kilei hakerem that became mixed in among permitted plants: it may not be picked, because what is attached to the ground is not nullified, and the one who picks it is the one causing the nullification in one part in two hundred and one - and one may not nullify a prohibition deliberately. After the fact, if he picked it without intending to, it is nullified in one part in two hundred and one. Rabbi Yosi permits it even at the outset, because a person does not render his entire vineyard forbidden on account of one single thing that was planted in it, and in an uncommon case the Sages did not decree.