The word itself
Ziran (自然) is two characters: zi, self, and ran, so or thus. Together: "self-so", or "so of itself". Modern Chinese uses the same word for "nature", which causes constant confusion, because the classical sense isn't the natural world at all.
It names a manner rather than a place. Something is ziran when it's the way it is out of its own tendency, without being pushed there. Water running downhill is ziran. Water in a pipe is not.
Chapter 25, and the puzzle in it
The famous sequence: humanity follows earth, earth follows heaven, heaven follows the Tao, and the Tao follows ziran.
Read as a hierarchy that's incoherent, since nothing is supposed to be above the Tao. The better reading, and the one the classical commentators take, is that the last step isn't a step at all. The Tao doesn't follow something further up. It follows its own self-so-ness. It has no model, no plan, and nothing it's trying to achieve.
Wang Bi, the third-century commentator, is blunt about it: ziran is a term for that which has no term. The chapter runs out of ladder on purpose.
How it relates to wu wei
They're two sides of one thing, and it's worth keeping them straight.
Ziran describes the state: things being as they are of themselves. Wu wei describes what you do so as not to wreck it: don't force, don't contrive, don't interfere.
Wu wei is the method and ziran is the condition it protects. A gardener practising wu wei produces a garden that's ziran. A gardener with topiary shears produces something else, which may be lovely and isn't this.
Why it isn't an argument for doing nothing
Ziran doesn't mean untouched. The classical texts are full of skilled human activity: Cook Ding's blade, the wheelwright, the swimmer in the waterfall. What makes those ziran is that the person has stopped imposing and started moving with the thing's own structure.
So a well-made chair can be ziran and a badly made one can't, which is a stranger and more useful idea than "leave nature alone". Our guide to the uncarved block comes at the same point from the material side, and wu wei versus laziness deals with the lazy misreading directly.
Reading further
Chapters 17, 23, 25, 51 and 64 carry most of the ziran material. Our core ideas guide places it among the book's other central terms, and 365 Days of Tao works through the complete text one verse a morning with commentary from Wang Bi and the other classical readers.
Common questions
What does ziran mean?
Literally 'self-so' or 'so of itself'. It names things being the way they are out of their own tendency rather than because something pushed them there. Modern Chinese uses the same word for 'nature', which is misleading.
What does 'the Tao follows ziran' mean?
That the Tao has nothing above it to follow. It follows its own self-so-ness: no model, no plan, nothing it's aiming at. Wang Bi called ziran a term for that which has no term.
What's the difference between ziran and wu wei?
Ziran is the condition, things being as they are of themselves. Wu wei is the conduct that avoids wrecking it. Wu wei is the method; ziran is what it protects.
Does ziran mean leaving nature untouched?
No. Skilled human work can be ziran when it moves with a thing's own structure rather than imposing on it. Cook Ding carving an ox is the classic example.
Which Tao Te Ching chapters discuss ziran?
Chapters 17, 23, 25, 51 and 64, with chapter 25 carrying the famous line about the Tao following ziran.