Nedarim, Chapter 3, Mishnah 1. The Mishnah opens: "Arba'ah nedarim hitiru chachamim" - there are four types of vows that the Sages declared permitted. The meaning is that the vow does not take effect at all, or that the one vowing does not intend at the time he says it for it to take effect.
The four vows are:
Nidrei ziruzin - vows made in order to prod another person.
Nidrei havai - vows stated in exaggerated language.
Nidrei shegagos - vows a person makes by mistake.
Nidrei onasin - vows made under circumstances beyond his control.
Let us now explain precisely what the Mishnah means in each of these cases.
Nidrei ziruzin, how so:
The Mishnah presents a case of someone selling an item to his fellow. The seller says: "Konam she'eini pocheis lecha min hasela" - I hereby make a vow, and 'konam' is a substitute term for a korban, that the item should be forbidden like a korban, and I am not willing to accept less than a sela for it. The buyer responds: "Konam she'eini mosif lecha al hashekel" - I hereby vow that I will not pay more than a shekel, which is half the sum you named.
To understand the case, one needs to know the relationship between the coins: every sela consists of two shekels, and of four dinars. Each shekel is two dinars, and a sela is four dinars. So the seller is saying that he will not come down from a sela, which is four dinars, and the buyer is saying that he will not pay more than a shekel, which is two dinars.
In truth, however, both of them want three dinars, and that is their intention from the outset - to compromise on a price of three dinars. Since there was no intention here to make a binding vow, they may compromise on three, and they are not violating their vows, even though it would appear on the surface as though each of them had vowed not to budge from the price he named.
Rabbi Eliezer ben Yaakov adds another example of nidrei ziruzin: someone who wants to prod his fellow to eat at his home, and tells him that all his property should be forbidden to him if he does not eat with him - this too is an example of nidrei ziruzin.
The continuation of the Mishnah - the declaration annulling vows:
The Mishnah continues with something that appears to have no connection to what came before, and indeed that is the case. The Gemara explains it as an entirely new idea. The Mishnah uses the wording "he says" - any vow he will make in the future should be void and meaningless, provided that he remembers at the time of the vow. The Gemara explains that words are missing here, and this is what is called 'chisurei mechsera'. The text of the Mishnah should read: one who does not want any of his vows to take effect over the course of the entire year - and since the Mishnah is dealing with vows that do not take effect, this law was taught here - should stand up on Rosh Hashanah and say, and here the words of the Mishnah begin: "Every vow that I will make in the future should be void," provided that he remembers the condition he stipulated at the time that he makes the vow.
The Ran writes that this is in fact the source for saying 'Kol Nidrei' on Yom Kippur. And although the Mishnah stated that one should stand up on Rosh Hashanah and make this declaration, the truth is that it can be said at any time; it is simply that the practice developed to do so at the beginning of the year, since a person wants all the vows of the entire year not to take effect. This is a way of ensuring that his future vows will not take effect.
"Uvilvad sheyehei zachur bish'as haneder":
In the Gemara we find two opinions in understanding this condition: does the Mishnah mean that the stipulation is effective on condition that he remembers at the time of the vow, or specifically that he does not remember that he made such a stipulation?
Abaye - it must mean that the stipulation is effective on condition that he does not remember at the time he vows that he made a stipulation, for if he did remember the stipulation and nevertheless vowed, he is seeking to nullify the stipulation. Therefore the meaning is: provided that he does not remember.
Rava - one can say that the meaning is that he does remember at the time of the vow that he made such a stipulation. However, he does not remember precisely which types of vows he intended the stipulation to apply to, and therefore whatever was in his mind then remains in force - that he wants that stipulation to stand, and that his vows should not take effect in the future.
In summary: In this Mishnah we learned the four vows that the Sages permitted - ziruzin, havai, shegagos and onasin; we examined nidrei ziruzin in the context of buying and selling, where both sides intend from the outset to compromise, and the addition of Rabbi Eliezer ben Yaakov; and also the declaration annulling vows on Rosh Hashanah, which is the source for 'Kol Nidrei', and the dispute between Abaye and Rava in understanding the condition "Uvilvad sheyehei zachur bish'as haneder".