Despite appearances at first glance, our present mishnah is quite simple to understand. Since in the previous mishnah we learned about women who do and do not drink the cursing waters and who do and do not collect the money of their Ketubah - our present mishnah continues with the same topic.
2:
The Torah grants the husband the unique right to divorce his wife at any time (and for any reason). The sages were extremely concerned that this left all wives extremely vulnerable from the financial point of view. (We must bear in mind the economic situation of women in ancient Israel: they were usually entirely dependent on their menfolk for economic sustenance - to begin with their father and then their husband.) Some 2500 years ago a process began which tried to rectify this situation, or at least to ameliorate it. It was to take 1500 years before Rabbenu Gershom, in Germany, was to bring the matter to its present 'solution' which makes mandatory a woman's consent to her own divorce. Sometime during the 2nd century BCE it became the norm that before the marriage the groom must give his bride a deed which is, in fact, an IOU. He mortgages all his property, and any property that he may come into in the future, to paying his wife a stipulated sum in the event of his predeceasing her or should he divorce her. (The amount stipulated was usually the result of negotiations between the parents of the happy couple.) Thus, should a man divorce his wife he must make available to her the sums promised at the time of the marriage.
3:
Under these circumstances a sense of equity would require that if the woman herself was the cause of the breakdown of the marriage, the hapless husband should not have to pay out the monies originally promised. The first situation detailed in our present mishnah is such: if a woman admits her infidelity, and her husband must then divorce her, she can hardly expect him also to pay out the money stipulated in her Ketubah. The same would apply if there were legally competent witnesses to her infidelity, whatever she herself might say. A third situation is one where the woman herself adamantly refuses to drink the 'cursing waters': given the social mores of the time this could be seen as tantamount to an admission of guilt.
4:
On the other hand, she should be entitled to her Ketubah money if it was an action of her husband that created the situation: if her husband changes his mind and refuses to let her drink the potion she is entitled to collect her Ketubah. If he charges her with infidelity but copulates with her on their journey to the Great Sanhedrin in Jerusalem she is also entitled to collect her Ketubah money. [For further elucidation see what we said in Paragraph 5 on Chapter One, Mishnah Three. The file is available in our archives.]
5:
The last situation discussed in our mishnah is where the husband has formally charged his wife with infidelity but before the ordeal can take place in Jerusalem he dies. Is his widow now entitled to the money in her Marriage Deed or has she forfeited the money by virtue of having been charged with infidelity? - bearing in mind that had he wished to her husband could have divorced her without charging her and she would then have been entitled to her money. Our mishnah states that in such a situation the woman is not required to undertake the ordeal. This is achieved by hermeneutic interpretation: the Torah [Numbers 5:15] specifically says that "the husband shall bring his wife to the priest" - the husband and no one else; since he is dead he cannot do so and she is excused.
6:
There is a maĥloket [difference of view] Between bet Hillel and Bet Shammai concerning her eligibility to collect her Ketubah money in such a case. The Gemara [Sotah 25a-b] explains the maĥloket as follows: Bet Shammai are of the opinion that the woman is holding her husband's IOU, as it were, and since her guilt has not been proven there is no reason why she should not present the deed for payment (from her late husband's heirs) as anyone else would. Bet Hillel are of the opinion that the fact that her husband charged her with infidelity is sufficient reason to assume that he would not have made these monies available to her had he lived.