At the end of the previous Mishnah we learned the ruling of Rabbi Yehudah: if a woman inherited property after her marriage and then sold it or gave it away as a gift, her act is of no effect. Only regarding property that came to her before the marriage did Rabbi Yehudah rule that the sale is valid after the fact, and even in that case the outcome is not to her advantage.
Our Mishnah brings the opinion of Rabbi Shimon, who distinguishes between two categories of property:
"Nechasim ha-yedu'im la-ba'al" - property known to the husband: ideally she should not sell it, and if she did sell it or give it as a gift, the sale does not take effect. The husband can annul it and return the property to his possession.
"Nechasim she-einam yedu'im la-ba'al" - property not known to the husband: here too she should not sell it in the first place, just like the other property; but since the husband does not know about it, if she sold it or gave it as a gift, the sale takes effect and he cannot annul it.
How do we define 'known to the husband' and 'not known to the husband'?
Rabbi Yosi son of Rabbi Chanina: 'Known to the husband' means real estate; 'not known to the husband' means movable items, which by their nature come and go.
Rabbi Yochanan: He disagrees and holds that both categories alike can be considered known property. 'Property not known' refers to property she inherited in a place far from where she lives, so that she may well receive word of it while the husband is entirely unaware of it. But property that he is aware of - selling it or giving it as a gift is not valid.
In summary: Rabbi Shimon distinguishes between property known to the husband, whose sale or gift is void and which the husband can take back, and property not known to him, where her actions stand. The Gemara records a dispute about how to define this division: according to Rabbi Yosi son of Rabbi Chanina the line is drawn between land and movable items, and according to Rabbi Yochanan between property within his knowledge and distant property that is not within his knowledge.