This mishnah addresses the question of a time limit on collecting the kesubah: is a widow given a set period in which to collect her kesubah from the heirs, or may she collect it forever? The law depends on where she lives after her husband's death - in her father's house or in her late husband's house.
The opinion of Rabbi Meir in the name of Rabban Shimon ben Gamliel:
"Kol zeman she'hi b'veis aviha" - a widow who does not remain in her late husband's house but returns to her father's house, where the heirs support her, receives nothing from them beyond her maintenance payments. Therefore there is no time limit on collecting her kesubah, and she may collect it forever, even after many years.
"Kol zeman she'hi b'veis ba'alah" - a widow who remains in her late husband's house becomes part of the household there and makes use of all the items found in the house. Therefore she must collect her kesubah within twenty-five years, and after that it is too late to collect it.
The reason, in the words of the mishnah: over the course of twenty-five years she does enough favors to offset the value of her kesubah. A woman living in the house does favors and gives things to her neighbors, and these tiny items accumulate over the years until they equal the sum of her kesubah. It turns out that she has already received her kesubah in practice.
The opinion of the Sages:
The Sages disagree and rule the opposite:
"Kol zeman she'hi b'veis ba'alah" - she may collect her kesubah forever.
"Kol zeman she'hi b'veis aviha" - she may collect her kesubah only up to twenty-five years, and if she has not collected within that time, it is too late.
The basis of their approach: the Sages were not at all concerned about the small and trivial items she makes use of, and these are not enough to take away her right to collect the kesubah. Their concern was something else entirely - that prolonged silence about collecting is a sign that she has forgiven her kesubah and waived it.
With this, both rulings fall into place: as long as she remains in her husband's house and lives together with the heirs, her silence is no indication that she has forgiven the debt, since she feels a sense of shame before them, and they honor her and treat her well, and she does not wish to burden them with a claim for the kesubah. But when she lives in her father's house, she is not in constant contact with them, they do not honor her, and she feels no embarrassment about demanding what is hers - and therefore her silence for twenty-five years proves that she has forgiven it.
If the widow died - the law regarding her heirs:
If the widow died and her sons, her heirs, wish to collect her kesubah, the mishnah states: "Yorsheha mazkirin kesubasah ad esrim v'chamesh shanim". Just as she herself had to demonstrate within twenty-five years that she had not forgiven the kesubah, so too the obligation rests on her heirs: from the time of her death they have twenty-five years to collect the kesubah. And if they do not wish to collect it within that time, for whatever reason, they must mention the matter and declare that the kesubah is coming to them and that they are not forgiving it - and in this way they secure their ability to collect it even after twenty-five years.
In summary: the Tannaim disagree about the time limit for collecting the kesubah. According to Rabbi Meir in the name of Rabban Shimon ben Gamliel, in her father's house she may collect forever, and in her husband's house only up to twenty-five years, because of the small favors that have accumulated to offset her kesubah. According to the Sages the law is the reverse, since the only concern is forgiveness of the debt based on silence - and in her husband's house her silence stems from embarrassment and honor and does not indicate forgiveness. And if she died, her heirs must mention her kesubah within twenty-five years in order to preserve the right to collect it.