As is well known, although nowadays both stages of marriage - the kiddushin (betrothal) and the nisuin (marriage itself) - are performed at a single ceremony, in earlier times the kiddushin was done first, and a long period could pass before the nisuin, sometimes even years. Against this background, our Mishnah discusses the preparation time granted to a couple in advance of the marriage.
The preparation time set by the Sages:
Besulah - a woman who has never been married is given twelve months from the time the husband, who has already betrothed her, demands that she marry him. During this time she prepares herself and her dowry: her clothing, her linens, and everything she needs to ready for married life.
"Keshem shenosnin la'ishah kach nosnin la'ish lefarnes atzmo" - the man is given that very same period. If the woman claims that she does not need so much time, and he claims that he does, he has the right to take twelve months to prepare himself. For him the preparation relates mainly to the wedding itself rather than to a dowry, but the amount of time is the same.
"V'almanah - sheloshim yom" - a widow is given only thirty days from the time he demands that she marry him, since many of the necessary items are already in her possession, and there is no need for a full twelve months.
"Higia zeman v'lo nisu":
What is the law when the date set for the marriage has arrived and the couple has still not married? The Gemara explains the Mishnah as referring to a case in which the delay comes from the side of the groom, who for some reason is putting off the marriage. In such a case "ochlos mishelo" - the woman begins to be supported from his food and out of his pocket, since he is the one causing the delay, and from that date she was already supposed to be under his financial responsibility.
Eating Terumah:
The Mishnah adds "ocheles biTerumah" - if the groom is a kohen, she may even eat Terumah. By Torah law, once the kiddushin has taken effect she is already permitted to eat Terumah, but the Sages forbade it as long as she is still in her father's home. The Gemara gives two reasons for this prohibition:
She might give some of the Terumah food to her brothers and sisters, who are forbidden to eat Terumah.
The husband might discover in her a physical blemish that only comes to light after the marriage, in which case the marriage turns out not to be valid, and she is forbidden to eat Terumah.
But once the date set for the marriage has arrived and it is the husband who postponed it, he may give her Terumah, for neither concern applies any longer:
Regarding the first concern: at that point he designates a place for her to eat, since he has no interest in her giving any of his food to her brothers and sisters - including ordinary, non-sanctified food. And once he has designated a place for her, there is no longer any concern that she will feed Terumah to her brothers and sisters.
Regarding the second concern: by that point the husband has already inquired among her relatives and clarified whether she has blemishes or not, so we need not be concerned about this.
The dispute between Rabbi Tarfon and Rabbi Akiva:
Rabbi Tarfon says - "nosnin lah hakol Terumah": he may give her all of her food as Terumah. And when she is tameh, such as during her days of niddah or from some other tumah, when she is forbidden to eat Terumah, she should sell the Terumah and buy ordinary food with the proceeds. This applies specifically as long as they are not yet married, but once she is married the husband must supply her with ordinary food during her days of tumah.
Rabbi Akiva says: since he is supporting her even before the marriage, he must give her half in ordinary food, which she will use during her days of tumah, and half in Terumah, which she will eat during her days of purity - just as a married woman does.
In summary: in this Mishnah we learned the preparation periods set by the Sages in advance of marriage - twelve months for a woman who has never been married and for a man, and thirty days for a widow; the law that when the date arrives and the husband causes the delay, the woman is supported from his food and may even eat Terumah; the two reasons for the prohibition against eating Terumah while in her father's home and how they are resolved; and the dispute between Rabbi Tarfon and Rabbi Akiva as to whether she is given all Terumah or half ordinary food and half Terumah.