We are learning Mishnah 2 in the fifteenth chapter of Maseches Yevamos. In the previous Mishnah we learned the halachah that a woman who traveled overseas with her husband and returned alone, when there was peace between them and peace in the world, and she says that her husband died - she is believed, and she is permitted to remarry.
The reservation of Beis Hillel:
In our Mishnah it emerges that Beis Hillel limited this halachah, at least initially. These are their words:
"Lo shamanu" - we did not receive this halachah, that a woman is believed to say that her husband died.
"Ela ba'ba min haketzir" - except in the case of one who comes from the wheat harvest that was taking place.
"U've'osah medinah" - in that same area and in that very same province.
"U'chema'aseh shehayah" - and in precisely the same circumstances as the incident that occurred.
What was that incident?
The Gemara explains that ten men went out to harvest wheat, and a snake bit one of them. His wife came and reported this to beis din, and beis din sent to investigate and found that her words were accurate. According to Beis Hillel, then, the credibility is restricted to that limited case alone, where it was possible to verify exactly what had happened; only there and in those circumstances do we not suspect that she may be lying. But if she came from overseas, from across the sea, she is not believed.
The answer of Beis Shammai:
Beis Shammai answered them that this is not so, and it makes no difference under what circumstances she brought the report:
"Echad haba min haketzir" - whether he came from the grain harvest.
"Ve'echad haba min hazeisim" - or whether he came from the olive picking.
"Ve'echad haba min habatzir" - or whether he came from the grape harvest.
"Ve'echad haba mimedinah limedinah" - or whether he came from one province to another.
And what is the basis for this? "Lo dibru chachamim baketzir ela baHoveh" - what you heard, that the Sages permitted it in a case where he came from the harvest, was not stated because the harvest is an indispensable condition, but because that is what actually happened. The Sages spoke of the common case and the typical situation, not of a requirement for the leniency.
The conclusion of the Mishnah:
Therefore, "chazru Beis Hillel lehoros k'divrei Beis Shammai" - Beis Hillel retracted, and began to rule and decide the halachah in accordance with the opinion of Beis Shammai, that the woman is believed even when she comes from overseas.
In summary: In this Mishnah we learned the dispute between Beis Shammai and Beis Hillel regarding the parameters of a woman's credibility concerning her husband's death. Beis Hillel limited the credibility to the circumstances of the incident that occurred alone - one who comes from the harvest and within the same province; Beis Shammai disagreed and established that this is not a condition but a description of the common reality, "lo dibru chachamim baketzir ela baHoveh", and therefore the credibility applies under any circumstances. And in conclusion, Beis Hillel retracted and ruled in accordance with the words of Beis Shammai.