Nedarim, Chapter 10, Mishnah 5. Our Mishnah deals with the stage between betrothal and marriage: once a certain amount of time has passed, the betrothed husband becomes obligated in the woman's support even before the marriage has taken place. The question before us is whether at that point the husband may annul her vows on his own, without the father, even though she has not yet entered the marriage.
The words of the Mishnah:
"Bogeres sheshahasah shneim asar chodesh" - a bogeres, a girl who has reached full maturity, is no longer under her father's authority. In earlier times it was customary for several years to pass between the kiddushin and the marriage, and at a certain point the betrothed husband would declare that the time had come to bring her into his home. From that moment she is given twelve months to prepare herself.
"Ve'almanah sheloshim yom" - in the case of a widow, once the betrothed husband demands to marry, she is given thirty days to prepare.
The Gemara discusses how this plays out in the case of a bogeres: does it mean that the kiddushin took place on the very day she became a bogeres, and at that very moment the betrothed husband demanded to marry, and the twelve months are counted from there.
In any event, once those days have passed and the marriage has not taken place, the husband becomes obligated in her support even without marriage. In this situation the Tannaim disagree:
"Rabbi Eliezer omer: ho'il uva'al chayav bimzonoseha - yafer" - since the obligation of support has already fallen upon him, he may annul her vows on his own, even though no father is involved in the matter, for she is already a bogeres and is not under his authority.
"Vachachamim omrim: ein haba'al meifer ad shetikanes lirshuso" - the obligation of support in itself is not enough, and the husband does not annul her vows until the marriage actually takes place and she truly enters his authority.
In summary: our Mishnah deals with a bogeres who has waited twelve months and a widow who has waited thirty days from the time the betrothed husband demanded to marry, at which point the husband is already obligated in her support even without marriage. According to Rabbi Eliezer, the obligation of support grants him the power of annulment on his own; according to the Sages, the husband does not annul her vows until she enters his authority through an actual marriage.