Notice the Ying Yang at the center. Yin and Yang is used to describe how opposing forces in the cosmos are connected and related. Each of the symbols surrounding the Ying Yang has 3 lines, either broken (Yin) or unbroken (Yang) - that's why they're called 'trigrams'. The 'I Ching' is supposed to be all the possible pairs of these trigrams: each pair is called a hexagram. Qian is Heaven; Dui is Lake; Li is fire; Zhen is thunder; Xun is wind; Kan is water; Gen is mountain; Kun is earth. This Yin and Yang is produced by Taiji ("great ridgepole": ridgepoles are horizontal beams in which rafters are fastened, along the ridges of roofs), meaning The Absolute, which itself is produced by Wuji ("without ridgepole"), meaning Without Limit, Boundless. By analogy, these poles are supposed to describe either magnetic (like the North Pole), geographic (like the so-called 4 corners of the Earth), or celestial (Poles located at the outermost reaches of the universe). Yin and Yang then produce 'lesser yin, greater yin' (the moon) and 'lesser yang, greater yang' (the Sun). These 4 phenomena (lesser yin, greater yin, lesser yang, greater yang) all act on the 8 trigrams, which leads to the possible pairs of the trigrams (the hexagrams, since instead of 3 stacked lines, there are 6): the I Ching, meaning 'The Book of Changes'.These hexagrams were recorded in Lian Shan (trans. "continuous mountains").
I'll give one example of a Hexagram as found in the I Ching. It is 6 stacked, unbroken (Yang) lines. It's name is 'qian', meaning Force, or possibly 'god'. Both the inner (lower) and the outer (higher) trigram mean Force, and that's why all the lines are unbroken (Yang).
