Meaning: This sentence describes the movement of a dog running quickly across a field.
Choice of Words: The verb "ran" means to move quickly on foot. "Dog" is an animal that is often kept as a pet. "Fast" means moving quickly. "Across" means from one side to the other. "Field" is an area of land used for farming or playing sports. When you put them together, the sentence means that a dog moved quickly on foot from one side of a field to the other.