What the I Ching is about

The I Ching is mainly about divination and The Heavenly Stems and Earthly Branches.

The I Ching is one of the three most curious books of ancient China, and a book of dialectical philosophy, which expounds on the worldly phenomena of heaven and earth. Originally, there were three I Ching books: Zhou Yi, Gui Zang and Lian Shan, but only Zhou Yi survives in the world, and both Gui Zang and Lian Shan have been lost.

Extended Information:

Many people think that the I Ching is the Zhou Yi and the Zhou Yi is the I Ching. The I Ching and the Zhou Yi are subordinate to each other, and the Zhou Yi is one of the "Three Yi" in the I Ching. The I Ching is regarded as the first of all the sutras, and is known as the general program of traditional Chinese culture.

For reference: Baidu Encyclopedia - I Ching