This is Mishnah 2 in chapter 13 of Maseches Yevamos, in which we continue to deal with the concept of mi'un (refusal).
"Eizohi ketanah shetzerichah lema'en?":
The Mishnah opens with a question: "Eizohi ketanah shetzerichah lema'en?" - under what circumstances is a rabbinic marriage significant enough to require mi'un, meaning an explicit statement by the minor girl that she does not want this husband and does not want the marriage. The Mishnah distinguishes between two cases:
"Kol shehisi'uha imah ve'echiha leda'atah" - her mother or her brothers married her off with her consent, at a time when she knows about it and is aware of what is happening. In such a case she requires mi'un.
"Hisi'uha shelo leda'atah - einah tzerichah lema'en" - if they married her off entirely without her knowledge, she does not need mi'un, and it is as though nothing had happened at all.
The view of Rabbi Chanina ben Antigonus:
Rabbi Chanina ben Antigonus says: "Kol tinoket she'einah yecholah lishmor kidusheha - einah tzerichah lema'en" - a girl so young that she does not know how to safeguard the item she received for her betrothal, whether it is money or a document, does not require mi'un, since she is too young to be married even by rabbinic law.
The Rambam defines which minor girl requires mi'un: she must be between the ages of six and ten, and within these ages we examine her level of understanding. If she knows how to distinguish between a document or money and fruits or nuts, she is considered to have understanding; and if she lacks understanding, she does not require mi'un at all.
The view of Rabbi Eliezer:
Rabbi Eliezer disagrees with the very concept of a rabbinic marriage for a minor, and says: "Ein ma'aseh ketanah kelum, ela kimefutah" - the betrothal act of a minor girl amounts to nothing, and even if she has understanding and knows how to safeguard the item of her betrothal, the marriage is not regarded as a genuine one; rather, she is treated merely as one who was seduced. Nevertheless, even according to his view she requires mi'un, and it is only with regard to all other matters that her marriage counts for nothing in halachah.
From this, two halachos in the Mishnah follow according to Rabbi Eliezer:
"Bas Yisrael leKohen - lo tochal biTerumah" - a Yisrael's daughter who married a Kohen is not permitted by this marriage to eat Terumah. Admittedly, all agree that she may not eat Terumah of Torah law, since she is a Kohen's wife only by rabbinic law, but seemingly she should have been able to eat rabbinic Terumah; according to Rabbi Eliezer, however, she may not eat even that.
"Bas Kohen leYisrael - tochal biTerumah" - a Kohen's daughter eats the Terumah of her father's house until she marries a Yisrael. In this case, according to Rabbi Eliezer, she continues to eat the Terumah of her father's house, since she is not viewed as married to a Yisrael even by rabbinic law in a way that would disqualify her from her father's Terumah.
In summary: in this Mishnah we learned which minor girl's rabbinic marriage is significant enough to require mi'un - when her mother or her brothers married her off with her consent; and which minor girl is exempt from mi'un - when they married her off without her knowledge, and according to Rabbi Chanina ben Antigonus also when she does not know how to safeguard the item of her betrothal, and according to the Rambam's definition, when she lacks the understanding to distinguish between a document or money and fruits or nuts. We also examined the view of Rabbi Eliezer, that "ein ma'aseh ketanah kelum, ela kimefutah", and its implications for eating Terumah in the case of a Yisrael's daughter married to a Kohen and a Kohen's daughter married to a Yisrael.