、。
The car plowed into the grass, rattled along for a while, and then stopped.
Literal
Car [topic-は] grass-field [into-に] plunged, for-a-while rattling ran-and stopped [explanatory-のです].
A narrative vehicle-incident description. 飛び込む ('plunge into, jump into') is a compound verb 飛ぶ ('fly, leap') + 込む ('into') — here used of a car careening off-road into grass. しばらく ('for a while, for some time') marks an indefinite duration. ガクンガクン is a 擬態語 for a jerky, rattling, jolting motion — used for rough movement over uneven terrain, stopping-and-starting, or shaking. Often doubled for intensified effect: ガクン (a single jolt) → ガクンガクン (continuous jolting). The ending のです frames the narrative as explanatory — 'the thing is, the car did X.'