Having discussed in the previous mishnayos the question of the status of a woman who was intimate with an unidentified man, we now turn to the incident recorded in our Mishnah.
The incident in the Mishnah:
A young girl went down to draw water from the well, and tragically she was violated, and it is not known who the man was. This raises the question: is she fit to marry a kohen, and if she bears a child, will the child be fit?
The Mishnah rules: if the majority of the people of that city are fit - that is, Jews of unblemished lineage, whose daughters and widows are permitted to marry a kohen - then this girl too is fit to marry a kohen. The leniency rests on the principle of following the majority: presumably the man who was intimate with her was one of that fit majority.
The Gemara's addition - the caravan of wagons:
The Gemara explains that there was an additional factor in this case: it happened near the wagons of Tzippori, meaning that at that very time a caravan of Jews of unblemished lineage from outside Tzippori was passing by.
It emerges that we have here what is called 'trei rubei' - two majorities:
The majority of the city: most of the city's inhabitants are Jews of unblemished lineage.
The majority of the caravan: most of the members of that group passing by were also Jews of unblemished lineage.
From here we learn that in matters of lineage there is a higher standard and a stricter demand: one majority is not enough, rather 'trei rubei' are required in order to permit this girl who was intimate with an unidentified man, and to establish that a Jew of unblemished lineage was indeed the one who was with her - and therefore she is fit to marry a kohen.
In summary: in our Mishnah we learned the case of the girl who went down to draw water from the well and was violated by an unidentified man, and the leniency that rests on the majority of the city's inhabitants being of unblemished lineage. The Gemara added that the incident occurred near the wagons of Tzippori, and from this we see that 'trei rubei' were required here - the majority of the city's inhabitants and the majority of the caravan - teaching us that in matters of lineage a stricter standard is required than in other prohibitions.