Phin's poem begins with a child's simple idea: the moon is following the car. Sitting in the back seat, the young speaker doesn't question it, and even notices the moon looks a little dirty, with its gray patches and the stars trailing behind. Later comes the explanation. Those patches are craters, and the stars are millions of light years away.
The poem then shifts from curiosity to feeling. The speaker remembers playing hide and seek in moonlight until bedtime and a goodnight kiss, a warm, safe memory. Growing up, though, the speaker looks at the same sky and cries, overwhelmed by how limitless the universe is. The moon and stars will still be shining after the speaker is gone, and the line "I was as fragile as glass" captures that sudden sense of smallness.
The mood moves from gentle and nostalgic to awed and a little sad. The closing couplet flips the opening: the moon was never following the speaker. The speaker was following the moon, which suggests a lifelong pull toward something much bigger than any one life.