Entailment, as defined by Webster's Collegiate Dictionary, refers to the restriction of property by limiting the inheritance to the owner's lineal descendants or to a particular class thereof. Although it was feudal in origin, an entail was legal device still used in Austen's time to prevent a landed property from being broken up, and from descending in a female line. The law was simply an extension of the practice of leaving the bulk, if not all, of one's wealth to one's heir, the eldest son.
Thus, entailed property was usually inherited by male primogeniture (by the nearest male-line descendant of the original owner). For example, Sir Elliot is the heir to Sir Walter in Persuasion. The law also prevented a father from disinheriting his eldest son, as the law prescribed that the son was his rightful heir. Furthermore, women only inherited if there were no male heirs left, and if there was more than one daughter, then they were all equal co-heiresses, and the land was subdivided evenly amongst them all. This subdivision is the cause of Mrs. Bennet's worry in Pride and Prejudice. She realizes that if her husband, Mr. Bennet dies, then she and her five daughters' would have to all live off the family's one estate that generated little income and their standard of living would fall considerably. Mrs. Bennet, therefore, urges her daughters to marry wealthy husbands before their father dies. In addition, if an heiress married, then she would be inherited by her sons, and the land would be transmitted along her husband's male-line. Austen expected her readers from her time to understand and sympathize in the Bennet daughters', and women's in general, pitiful predicament.
The basis for the law of entailment was that ownership of land was not simply an ornament to the family. Rather, it was the foundation of its aristocratic position; it is what made the family noble and enabled it to live the way it did. The land produced a steady income that freed the family from the need to labor and allowed it to live a refined and potentially idle life. Hence, real estate ownership was much more meaningful than the regular possession of other assets or even cash. The estate lent status to the entire family as long as it lasted.
Landowners, like Sir Walter, were therefore very intent on keeping their estate whole. As in Persuasion, they recognized the threats of subdivision amongst females or the dissipation of the land by sale, if the owner had to sell the land to raise funds and then continue to lose the proceeds. The whole family would then lose social status.
Another detail of the law was that entails were periodically renewable and even breakable with the consent of an heir who had come of age. Similarly, "instead of pushing his fortune in the line marked out for the heir of the house of Elliot, [Sir Elliot] had purchased independence by uniting himself to a rich woman of inferior birth" (Persuasion p. 28).
The law has progressed since Austen's time. Today in England the owner is permitted to convey the entailed land by a simple deed or even by will.