Shevuos, Chapter 1, Mishnah 2. This mishnah opens with a discussion of the concept of awareness of tumah: a person who knew that he was tamei and then forgot it, and ate kodashim or entered the Beis HaMikdash - and as we mentioned, he is obligated to bring a korban.
"Kol sheyeish bah yediah bat'chilah viyediah basof v'helem beintayim" - any case in which there is awareness at the beginning, awareness at the end, and a lapse of awareness in between:
The mishnah establishes that someone who had awareness at the beginning - he knew from the outset that he was tamei - and awareness at the end - he discovered afterward that he had been tamei - and in between there was a helem, meaning that he forgot that he was tamei. As we saw in the previous mishnah, he does not necessarily have to forget the tumah itself: he could forget that this place is part of the Beis HaMikdash, or that this meat is kodashim. Either way, this is a helem, a lapse of memory.
In such a case - "harei zeh b'oleh v'yored" - he is obligated to bring a korban oleh v'yored. A korban oleh v'yored is an offering that varies according to the financial means of the person bringing it:
A wealthy person: brings a chatas from an animal.
A poor person: brings a chatas from a bird offering.
The poorest of the poor: a person of very limited means brings only a tenth of an eifah of fine flour, and that suffices for his chatas; it is called minchas chotei.
Let us emphasize once more: a korban oleh v'yored is brought only when there was awareness at the beginning and awareness at the end.
Awareness at the beginning without awareness at the end:
The mishnah now turns to a case in which there is awareness at the beginning but no awareness at the end: he knew at the outset that he was tamei, but he never afterward discovers that he ate kodashim or that he entered the Beis HaMikdash while tamei. The knowledge never reaches him at all. How, then, will he receive atonement?
To this the mishnah says: "sa'ir hana'aseh bifnim v'yom haKippurim toleh" - the goat, the chatas offering whose blood is brought into the innermost chamber, the Kodesh HaKodashim, on Yom Kippur, together with Yom Kippur itself, provide a suspension. This is a temporary measure that grants him atonement in the interim, until it becomes known to him, until he eventually discovers that he was tamei, and then he brings a korban oleh v'yored.
The function of the suspension:
The Gemara discusses what the function of this suspension is and how it helps him in the meantime. If it is not going to atone, and it does not truly atone - since in the end he will bring a korban afterward - what does it accomplish for him?
At first the Gemara wants to say that if he dies, he will die without any sin. But the Gemara rejects this: death itself would be an atonement. The Gemara therefore concludes that the suspension shields him like a shield and buckler against suffering - it protects him from receiving punishments in the meantime, from suffering and hardships that he would have received as punishment for this sin. This protection stands for him until he has awareness at the end, and then he brings a korban oleh v'yored.
In summary: In this Mishnah we learned the two situations of awareness of tumah: awareness at the outset together with awareness at the end, with a lapse of awareness in between, which obligates one in the sliding-scale offering in its three forms; and awareness at the outset without awareness at the end, in which the goat whose service is performed inside and Yom Kippur suspend the punishment, and this suspension shields him from suffering until awareness comes and he brings his offering.