They broke the entail

The oddest thing about Downton Abbey is that the whole plot revolves around the matter of the estate being entailed on the heir to the title and thus being unable to be passed to the daughters; and no dialogue was inserted to explain what an entail was. As a lawyer of the old school - well, as a legal graduate of the old school - I knew, but I can't really believe that the knowledge is so widespread in the general population that it can be assumed. I'd like to do a quiz.