Today it means: A movable enclosure in which chickens can scratch the ground for food, allowing them to be pasture-raised (benefiting from grass, insects, sunshine, and breathing space) and offering benefits to crop rotation systems (e.g., insect control, soil manuring). Full definition →
So called because it serves some functions of a tractor, such as weeding and fertilizing. Attestations from the 1970s exist, according to the metadata connected with snippet views in Google Book Search results.
Etymology from Wiktionary, CC BY-SA 4.0.