What is "chó sủa trăng" really means?

I have no idea. But from its context, I guess the dog is barking at the moon. Does it mean something like, a person is trying to say something important to the world, but he's so small and no-one would even care about it. That's how I interpret it.

Thank you for your explanation.

btw, the dog also does not know that the moon is not a living object and the moon obviously won't reply to the dog. Could it also means that a person is saying something but he is ignorant about?

tqt, I understand what you mean.

Yup, it means that you're trying to say something but noone is there to listen to you.