Two people lie on a rooftop at 3 AM, staring up at the night sky. They see a shooting star and make the same wish: that the other will never leave. It's a tender, hopeful moment, and the poem calls them "innocent," which tells us how sincerely they believe in it.
The last line changes the tone. The speaker knows what the two of them don't: shooting stars are things that "crash and burn," so the thing they trust with their wish is already falling apart. That suggests their love, or their hope of staying together, might end the same way. The speaker seems to feel a quiet sadness, maybe protectiveness, watching from a place of knowing more than they do. The mood starts out dreamy and close, then turns bittersweet and a little ominous. The late hour and the dark sky add to the sense of fragility, as if this perfect moment can't last.