A more thorough explanation:

Definition: Terms refer to the Year Books, which are books of cases reported anonymously and regularly. They cover the period from the reign of Edward I to the time of Henry VIII. The reports were probably originally prepared by law teachers and students and later by professional reporters or scribes. The title "Year Books" comes from grouping the cases under the regnal years of the sovereigns in whose reigns the reported cases were cited.

For example, the Year Books were written in Anglo-French, the language of courtly speech. Their authorship is unknown, and they are referred to by the generic name "year-books." The reports settled into a more uniform and at times apparently single series. The year-books did not end at any fixed date. What has usually been taken as their end is the result of two concurrent factors: the advent of printing, and the practice of identifying reports by the name of the author.

The Year Books are important because they contain ideas and suggestions that were used to teach law students. They were not used as historical authenticity as precedents.