The I Ching (Book of Changes) is a book of change drawn by two forces, yin and yang. Broken and solid lines stack six deep to form 64 situations (hexagrams), and the moving line among them reveals the flow from now to next.
From two lines to 64 hexagrams
The I Ching begins with just two: the solid line (yang ⚊) and the broken line (yin ⚋). Stack three and you get the eight base images, the trigrams; stack two trigrams and you complete a six-line hexagram, one of 64.
The 64 hexagrams symbolize 64 typical situations we meet in life. Drawing a hexagram means reading the situation you are in now against one of these 64.
The moving line — where change begins
The reason the I Ching is the "Book of Change" lies in the moving line. Among the six positions of the hexagram you draw, the changing one is the moving line, and when it flips the primary hexagram leads to a new one (the changing hexagram).
If the primary hexagram is "the situation now," the changing hexagram is "the direction it flows." So an I Ching reading gives not a flat answer but insight into where to move from where you stand.
Frequently asked
Rather than telling a fixed future, the I Ching reflects the present situation and the direction of change within it. The answer is closer to "how best to conduct yourself" than "what will happen."
The method changes with how many positions are moving. Traditionally there are several rules that decide, by the number and position of moving lines, whether to weight the primary or the changing hexagram.