In this song, she's metaphorically the moon, seeing everything her loved one does and she's just... always aware. She's looking over you, wishing that you see her too, even just glance at her beautiful sky. In the end she just wants to move on knowing that her feelings aren't reciprocated.
It's two friends, but one of them fell for the other. She does everything for him yet he doesn't register it as something romantic, or maybe he does, but he doesn't want her that way. She even sees her happy with someone else, but she wants him to know that she's in love with him... and yet he's turning a blind eye, hurting her more.