You’re lying down in the grass looking up when suddenly a pie comes floating in the sky! Cool! You’re probably just daydreaming or hallucinating. It’s a nice idea to think about though highly unlikely. That’s what this idiom is all about.
Something nice but unlikely to happen. An empty wish
>Turns out that the startup business she planned was just a pie in the sky
>Their plans to revive the club after playing dirty tricks are just pie in the sky