Don't Tell me the moon is shining, show me the glint of light on broken glass
What I think it means is that, Show proof that something is happening. Don't just go by word, Words lie more often than eyes do.
I think its a good quote, I can't find anything I don't like about it and it makes sense. I'd rather see for myself than just go by the word of someone else.
I think it connects to Composition by the fact that the teacher sees our work and can see how good (or bad) it is, no teacher is going to grade you just by your word, They have to see it with their eyes.