In the previous Mishnah we learned that the enactment of the Sages regarding kesubas benin dichrin - that each woman's sons inherit their mother's kesubah rather than dividing the estate equally - depends on one condition: that the estate contain a surplus of at least the value of a dinar beyond the two kesubos. Only when this surplus exists, to be divided equally among all the sons according to Torah law of inheritance, does the division of the kesubos take effect, even though it is not necessarily an equal division.
Property that is Available and Property in Hand:
What is the law when the father's property does not amount to the value of a dinar, but property classified as 'available' (ra'ui) is destined to come to the orphans, property that will eventually reach their hands? For example, their grandfather is still alive and the father died in his lifetime, and they know that when the grandfather dies they will be his heirs, and at that point the estate will exceed the value of a dinar. Since at present they have not actually inherited this property, they have no hold on it, and it is not counted as the surplus dinar.
Rabbi Shimon's Additional Condition:
Rabbi Shimon adds a further requirement: even if the estate contains more than the value of a dinar, that is not enough when the property in question carries no guarantee. There are two categories of property:
Property that carries a guarantee - land. Property from which a debt can be collected, and even if it was sold it can be collected from the buyer by someone who had a prior claim on it.
Property that carries no guarantee - movable goods, anything that is not land.
According to Rabbi Shimon, if all the estate contains is movable goods - "eino kelum" - it is nothing, and it is not counted at all toward the surplus dinar required for the equal division. The condition is fulfilled only "ad she'yehei sham nechasim she'yesh lahen achrayus yoser al shtei ha'kesubos dinar" - only if there remains in land a value of at least a dinar beyond the two kesubos.
In summary: the enactment of kesubas benin dichrin takes effect only when the estate contains a surplus dinar beyond the two kesubos. Property classified as 'available', which has not yet actually come into the orphans' hands, is not counted in this reckoning. And Rabbi Shimon adds that even the property already in their hands is not counted unless it is property that carries a guarantee - land and not movable goods.