Before we elaborate more on all of the halachot (laws) that come with swearing, the Mishnayot will take a brief pause and talk about a person entering the Bet HaMikdash (Holy Temple) while he was Tamei (ritually impure), or eating Korbanot (offering), eating Kodashim (holy food) while he was Tamei as well. The Mishnah will talk about it for this Perek (chapter) and the next Perek as well.
In general, a person could only bring a Korban that would atone for his Aveirah (sin). If the Aveirah was done unintentionally, if he did it by mistake, we'll talk about two different Korbanot that a person would bring, or two different types of Korbanot that would happen for two different situations of him unintentionally violating these mitzvot (commandments). The Mishnah says that if there was a situation where a person knew that he was Tamei before he sinned and knew he was Tamei after, but while he was actually sinning, at the time of the Aveirah, he forgot, and then he either went into the Bet HaMikdash during that time or ate holy food of the Bet HaMikdash during that time—he ate a Korban, for example—and then afterwards, after he finished everything, he realized that he couldn't have done that. He was Tamei when he did it. He's obligated to bring a Korban, a certain type of Chatat (sin offering), as a Kapara (atonement) for his sin.
In an oleh veyored, the price changes depending on what the person could afford. If he was wealthy, for example, he would bring a female sheep or a goat as Chatat. If he was poor, he would bring two birds. One would be a Chatat, one would be an Olah (burnt offering). If he was super poor, he would only bring a Mincha, a flour offering.
Another type of situation is Yesh bah yedi'ah batechilah ve'ein bah yedi'ah basof. If he knew that he was Tamei before the sin, but after the sin, he did not know that he was Tamei, and obviously, he also didn't know about the Tum'ah (ritual impurity) while he was doing the sin, so he can't really bring a Korban at that point because he doesn't even realize that he was Tamei. So he says, what happens to this person is the Se'ir LaShem. The goat. That's brought on Yom Kippur (Day of Atonement) by the Kohen Gadol (High Priest). Together with Yom Kippur itself, Itzumo shel Yom, the actual day of Yom Kippur, has a certain feeling of atonement that it gives to a person. Both of these suspend the punishment. They suspend the punishment this person is supposed to get until eventually he realizes that he made a sin. Hopefully he could backtrack and trace his step and realize, “Oh, you know what? I have to bring a Korban. I actually was Tamei at that time.” At that point, he would have the proper kapara that he would need.