A more thorough explanation:

Year Books are books of legal cases that were anonymously and regularly reported during the reign of Edward I to the time of Henry VIII. They were named "Year Books" because they were grouped under the regnal years of the sovereigns in whose reigns the reported cases were cited. The reports were probably originally prepared by law teachers and students and later by professional reporters or scribes.

The reports 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 earliest reports were used as a case-method of instruction in the law school which served the apprentices of the Bench before the emergence of the inns of court. The contemporary value of the earliest reports lay not in their historical authenticity as precedents but in the ideas and suggestions which they contained.

For example, if a judge in the 14th century wanted to know how a similar case was decided in the past, they would look to the Year Books for guidance. The Year Books were not only a source of legal precedent but also a way for judges to learn from the experiences of their predecessors.

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.