In the previous Mishnah we dealt with the offerings a person brings upon the completion of his nezirus. Mishnah 8 in chapter 6 of Maseches Nazir continues and deals with a case in which the nazir brought three animals for the offerings of his nezirus without specifying which animal was designated for which offering.
The three animals and the three offerings:
The three offerings of the nazir - chatas, olah and shelamim - come from three different types of animals, and from their very definition one can tell simply by looking at them which one is designated for which offering:
Chatas - a ewe in its first year, necessarily female.
Olah - a male lamb in its first year.
Shelamim - a ram, which is a mature male.
Therefore, the animal fit for the chatas - the ewe in its first year - is brought as the chatas; the one fit for the olah - the male in its first year - is brought as the olah; and the one fit for the shelamim - the ram - is brought as the shelamim. Normally, one who brings an offering is required to identify the animal and designate it for its particular purpose and offering, but here this is not necessary, since by default each animal is established for the offering that suits it.
Burning the hair beneath the pot:
The Mishnah describes the process in which the nazir takes the hair he shaved from his head and sends it to be burned beneath the pot in which the ram of the shelamim is cooked, as described in the passage in the Torah. Even if he shaved his head outside the Beis HaMikdash, in the "medinah" - meaning in Jerusalem - he still sends the hair to be burned beneath the shelamim.
When does this law apply? At the conclusion of a nezirus in purity, when the nazir did not become tamei. But a nazir who became tamei and must shave his head does not send his hair to be burned beneath the fire of the offering; this law was stated only regarding a pure nazir.
The view of Rabbi Meir:
Rabbi Meir disagrees and holds that any nazir who must shave his head sends his hair to be burned beneath the pot of the offering, and it makes no difference whether he performed the shaving in the Mikdash or in Jerusalem. Regarding a pure nazir this is explicit in the verse, that he sends the hair beneath the pot of the shelamim; and regarding a tamei nazir he sends the hair beneath the pot of the asham.
The only exception according to Rabbi Meir is when the two conditions are combined: a tamei nazir who shaved in the "medinah," outside the Beis HaMikdash - in which case he does not send his hair. But if he shaved in the Beis HaMikdash, even a tamei nazir burns his hair beneath the pot, beneath the pot of the asham offering.
In summary: In this Mishnah we learned that the three animals of the nazir's offerings are established on their own for their respective offerings according to their type, without any need for explicit designation; and also the law of sending the hair to be burned beneath the pot of the shelamim, which applies to a pure nazir even when he shaved in Jerusalem. Rabbi Meir extends the law to every nazir, and regarding a tamei nazir - beneath the pot of the asham, excluding only a tamei nazir who shaved outside the Beis HaMikdash.