Because the scripture of Zhouyi is profound and simple, scholars in the Spring and Autumn Period have found it difficult to read this book. So there began to appear words to explain the scriptures, which were called the Book of Changes in history, also known as the Great Biography of Zhouyi. In ancient times, "biography" had the meaning of explanation, and any document that explained and expounded the meaning of classic works could be called "biography".
Extended data
Generally speaking, when annotating the hexagrams in Zhouyi, Yi Zhuan paid attention to the annotation of the hexagrams, raised the specific hexagrams to the abstract relationship between Yin and Yang, sorted out and explained them from the meaning of the sixty-four hexagrams in Zhouyi, and revealed the relationships between hexagrams and hexagrams.
In this way, the sixty-four hexagrams in the Book of Changes have changed from disorder and impropriety to an organic and interrelated unity with certain logic. Through a new interpretation of the old formal language of Zhouyi, Yi Zhuan tells people's rational thinking about the universe and life in the new era, which proves that the ancestors grasped the thinking results of the world philosophically.
Baidu Encyclopedia-Yi Chuan