Chagigah, Chapter Two, Mishnah 5. From this point on, the tractate turns to the laws of tumah and taharah. The reason: toward the end of the tractate we will learn that during the festival season, on the three pilgrimage festivals, all of Israel is considered pure - even the unlearned among them - because they would purify themselves in honor of the festival, so that they could enter the Beis HaMikdash and eat from the meat of the offerings. Because of that discussion which is yet to come, the Mishnah already begins here with the laws of tumah and taharah.
The text of the Mishnah:
"Notlin la'yadayim l'chullin ul'ma'aser v'lit'rumah, ul'kodesh matbilin, ul'chatas - im nitme'u yadav nitma gufo" - one washes the hands for ordinary food, for ma'aser and for Terumah; for sacred food one immerses the hands; and for the chatas - if a person's hands became tamei, his whole body is tamei.
The underlying principle: the hands of a person who does not guard them from touching things, but simply uses them in the normal way, are called "ordinary hands," and their status is that of a sheini - a second degree of tumah. From here different levels of purity of the hands are derived, corresponding to the level of holiness of what a person is about to eat.
The three levels in the Mishnah:
"Notlin la'yadayim l'chullin ul'ma'aser v'lit'rumah" - before eating bread of ordinary food, and likewise bread of ma'aser sheini and bread of Terumah, washing the hands is sufficient.
"Ul'kodesh matbilin" - for eating the meat of offerings and the meal offerings, washing the hands is not enough; rather, one must immerse the hands in a mikvah. This is the only way to raise them to the higher level required for sacred food.
"Ul'chatas - im nitme'u yadav nitma gufo" - in matters connected to the parah adumah, which are called chatas, if a person's hands became tamei his entire body is considered tamei, and immersing the hands alone is not enough; rather, he must go down and immerse in a mikvah.
In summary: in this Mishnah we learned three levels in the purity of the hands: washing the hands for ordinary food, for ma'aser and for Terumah; immersing the hands in a mikvah for sacred food; and immersing the entire body in matters of the chatas. The basis of the halachah is that "ordinary hands" are considered a sheini for tumah, and the higher the level of holiness of what is eaten, the higher the level of purity required.
Further on in the tractate we will continue with the laws of tumah and taharah, until we reach the discussion of the purity of all of Israel during the festival.