The six line texts of the first hexagram track a dragon: hidden and not to be used, appearing in the field, active and wary at nightfall, wavering over the deep, flying in the sky, and finally arrogant, with cause for regret. That progression from concealment to overreach is one of the most economical things in the book, and it is the reason this hexagram is so often used to teach the whole system.
It also carries something almost unique. Hexagrams 1 and 2 have a seventh statement, "use nine" for this hexagram, "use six" for the second, which belongs to no individual line. What it is for has been argued over for two thousand years. The usual explanation is that it applies when every line of the reading is a changing line, so that the whole figure transforms into its opposite; other readings take it as a general remark about the hexagram's nature. The text does not say, and no early source settles it.
Wilhelm renders it as a flight of dragons without heads, an image of strength that does not insist on leading. Legge, in 1882, handles it more cautiously and flags the difficulty. Rutt treats it as a technical divination note. That spread of treatments over a single short phrase is a fair sample of what reading this book actually involves.
The practical lesson the dragon sequence teaches is about timing: the same force is right at one stage and disastrous at another, and nothing in the hexagram is good or bad independent of where in its course it is caught. That principle governs the whole book, and this is where it is stated most legibly.