Meaning: This sentence describes the dog's action of barking and the loudness of its bark.
Choice of Words: The word "dog" refers to a four-legged animal that is often kept as a pet. "Barks" is a verb that describes the sound made by a dog. "Loudly" describes the volume or intensity of the sound. When you put them together, "The dog barks loudly" tells us about the dog's action and the volume of its bark.