can you help me explain this sentense
I don't think I've ever heard "cry for the moon."
"Reaching for the moon" is a fairly common expression, though.
It means attempting the impossible.
Surprisingly, "reaching for the moon" feels somewhat positive. It can mean going after your dream, whatever the odds. I don't think you'd use it scornfully, to say that someone's efforts are useless, hopeless, without value.
cry for the moon = ask for the moon = make an unreasonable demmand, request the unattainable
Charles Dickens used the expression cry for the moon in his novel Bleak House.
There are also a song (symphonic metal band, Epika; see their debut album The Phantom Agony)and a film (by Dizzy Elmer) with this title .