Nedarim, Chapter 2, Mishnah 5. In the previous Mishnah we learned the rule "stam nedarim l'hachmir u'peirusham l'hakel": when a person makes a vow and compares his vow to something that can be understood in two ways - as a davar hanadur, an item forbidden by means of a vow, in which case the item compared to it also becomes forbidden; or as a davar ha'asur, an item already forbidden by the Torah, in which case the vow does not take effect - when the wording is left unexplained we follow the stringent side, and the item is considered forbidden. But when the one who vowed himself explains what he had in mind, we no longer follow the unexplained wording but his explanation, even though that explanation leads to a leniency.
Our Mishnah brings examples of this rule:
"Nadar b'cherem, v'amar: lo nadarti ela b'cherem shel yam" - he vowed using the word cherem and then said: I only vowed by a sea cherem. An unqualified cherem refers to a cherem dedicated to Heaven, which is a davar hanadur, and in that case we would rule stringently. But here the one who vowed explains that he did not mean a cherem with any halachic status, but rather a "cherem" in the sense of a fisherman's net.
"B'korban, v'amar: lo nadarti ela b'korbanos shel melachim" - he did not mean a korban offered on the altar, but the gifts presented to kings, which have no connection whatsoever to a davar ha'asur.
"Harei atzmi korban, v'amar: lo nadarti ela ba'etzem shehinachti li lihyos noder bo" - on the face of it he forbade himself to his friend like a korban, so that his friend may not derive benefit from him. But he explains that he did not mean his own body (atzmi), but rather the bone (etzem) that he had set aside beforehand precisely in order to vow with it, in a way that his words would sound like a vow while in fact there is no vow upon himself at all.
"Konam ishti neheinis li, v'amar: lo nadarti ela b'ishti harishonah shegirashti" - he did not mean the wife who is with him now, but his first wife, whom he already divorced.
In all these cases - "al kulan ein nish'alin lahem" - the one who vowed does not need to request annulment of his vow and ask a sage to release him, since, as we learned, "peirusham l'hakel," and his own explanation is enough to permit the matter.
And what is the law if he nevertheless comes to request annulment of his vow?
If that person, who explained his words from the outset, still comes to the sages and asks: "I made such and such a vow, but this is what I meant" - this shows that he is not sufficiently knowledgeable and is liable to act carelessly. Therefore, "v'im nish'alu, onshin osan u'machmirin aleihen" - if they did ask, we punish them and rule stringently with them.
And what is the punishment? If they were lenient with themselves and did not treat the item as forbidden, we obligate them to treat it as forbidden for the same period of time - say, twenty days - during which they permitted it to themselves. And likewise "machmirin aleihen," that we are not lenient with them to release their vow easily.
"Divrei Rabbi Meir" - this is the opinion of Rabbi Meir, who holds that we do not fully believe the unlearned, who are not expert in halachah, regarding the explanations they give for their words. Therefore we must be stringent with them, so that they not accustom themselves to vows of this sort.
The opinion of the Chachamim:
"V'Chachamim omrim: pos'chin lahem pesach mimakom acher" - the Chachamim do not accept his claim as an established fact. He does claim that there is no vow here, since he meant not the wife who is with him but his previous wife, yet the Chachamim assume that it was a valid vow, and they look for a different reason and opening for annulment: that had he known how difficult the matter would be, he would never have vowed. In other words, they treat him with a genuine annulment of vows.
"U'melamdin osan" - Rashi explains that this does not mean mere instruction, but actual rebuke. And the reason is, "kedei shelo yinhagu kalus rosh binedarim" - so that they not treat vows lightly, but rather relate to them seriously and understand that one must be very careful with the declarations that come out of one's mouth.