Before us is Mishnah 7 in the first chapter of Maseches Kesubos. To understand the dispute it records, we must return to Mishnah 3, where we learned of the disagreement between Rabbi Meir and the Sages regarding a mukas eitz - a woman who lost her signs of virginity through an injury alone, and not through relations:
Rabbi Meir: Her kesubah is two hundred, as is the law for a full virgin.
The Sages: Her kesubah is a maneh alone, one hundred, as is the law for a widow.
The difference between a mukas eitz and a non-virgin:
Even according to the Sages there is a fundamental difference between a mukas eitz and a woman who has had relations. If a man marries a woman on the assumption that she is a virgin and it turns out that she is not, this is a mistaken transaction, since the kiddushin was made on the basis of an assumption that was untrue, and she has no kesubah at all. But if it turns out that she is a mukas eitz, there is no mistaken transaction here: true, according to the Sages her kesubah is not two hundred but only one hundred, yet those hundred she receives in full.
The dispute in our Mishnah:
The case is a woman who is found not to have signs of virginity, and she claims: "Mukas eitz ani" - I never had relations with any man, and the loss of my virginity came about through an injury, and therefore my kesubah is due to me, and even according to the Sages I collect one hundred. The husband, for his part, claims: "Lo chi, ela derusas ish at" - you had relations with a man, and on that basis you have no kesubah at all.
Rabban Gamliel and Rabbi Eliezer say "Ne'emenes" - she is believed. Similar to their position in the previous Mishnah, she is believed to say that she is a mukas eitz and to collect a kesubah of one hundred dinars, since she holds a definite claim: she knows clearly what happened, whereas the husband claims only a possibility - he is saying no more than what may have happened. Under such circumstances we presume that the one with the definite claim is in the right.
However, as we mentioned in the previous Mishnah, a definite claim alone is not enough, and an additional chazakah is required in order to override the presumption of ownership on the husband's side:
In the previous Mishnah: she had the status of a virgin until that moment, and therefore we leave her in her established status until the latest possible time.
In our Mishnah: there is a chazakah that Jewish women do not make themselves available and do not have relations with men, and from here it follows that the matter did indeed come about through an injury and not through relations.
This chazakah joins together with the definite claim she holds, and thereby overrides the presumption of ownership, the fact that the husband is in possession of his money.
Rabbi Yehoshua disagrees and says: "Lo mipiha anu chayin, ela harei zo b'chezkas be'ulah ad shetavi re'ayah lidvareha" - we do not live by her word and we do not believe her. By force of the husband's presumption of ownership, since he is in possession of his money, she is to be presumed to have had relations, and therefore she does not collect her kesubah until she brings proof for her words.
In summary: in this Mishnah the Tannaim disagree about a woman who is found not to have signs of virginity and claims "Mukas eitz ani," while the husband claims "Derusas ish at." Rabban Gamliel and Rabbi Eliezer rule that she is believed, by force of combining the definite claim she holds with the chazakah that Jewish women do not make themselves available, which overrides the husband's presumption of ownership. Rabbi Yehoshua holds that the presumption of ownership stands in place, and therefore she is presumed to have had relations until she brings proof for her words.