A chicken tractor (or arc) is a cage that can be moved over a paddock or garden beds. It is half-way between a fixed chicken coop and permanent free-ranging.
The chickens have access to fresh pasture and insects, but are also confined to a certain area. The term “chicken tractor” was first used by Permaculture co-founder Bill Mollison to describe the effect of putting chickens in a small open-bottomed cage, and allowing them to till and fertilise the garden, instead of using a tractor.
Since then the meaning of the term has broadened.