A more thorough explanation:

Definition: Liberum maritagium (pronounced lib-uh-rum mar-uh-tay-jee-um) is a type of entailed estate where the donor (usually the father) retains control of the land by refusing to accept feudal services from the donee (usually the donor's daughter) for three generations. If the donee's descendants fail to maintain the estate during this time, the land returns to the donor.

Example: Let's say a father wants to provide for his daughter by giving her an estate. He wants to make sure that the estate stays in the family and doesn't fall into the hands of someone else. So, he gives her the estate with the condition that she and her descendants cannot perform feudal services for three generations. This means that they cannot pay homage or provide military service to the lord of the land. If the daughter's descendants fail to maintain the estate during this time, the land will return to the father or his heirs.

Explanation: Liberum maritagium was a way for fathers to provide for their daughters while also ensuring that the land stayed in the family. By retaining control of the land, the father could prevent it from falling into the hands of a collateral heir. The three-generation rule was considered sufficient to demonstrate that the line was well established and that the donee's descendants were capable of maintaining the estate. If they failed to do so, the land would return to the donor or his heirs.