The speaker looks back on a night that felt enchanted. Under the moon, with light shining in someone's eyes, they "took my spells for a run," as if trying out a little magic on a possible romance. The night seems full of promise, and the tone is almost playful.
Then daylight arrives and everything changes. The speaker guesses that the magic "faded away," and the casual "I guess" hides some hurt. The last three lines are short and plain: "you're not here, / And you're not mine." Nothing is explained, which makes the loss feel blunt. The final image of "a million ifs" standing in between suggests all the unanswered questions and missed chances that keep the two people apart.
The mood moves from dreamy to wistful. The moon stands for a glowing, make-believe world, while the sun brings reality. The speaker isn't angry, just left wondering what might have happened, and that quiet longing is what stays with the reader.