A person makes up a tune one morning in the shade, plays it on a pipe, and feels pleased when the birds and chipmunks seem to enjoy it too. The speaker is proud of this song, and that pride turns playful: they offer to play it for you, but not for free. The Beloved heard it as a loving gift beneath her moonlit window. Since you are not the Beloved, you must pay a silver shilling, round and shining like the moon. Copper is not enough for a song made out of nothing and so beautiful.
The mood is light, wry, and a bit tender. There is genuine delight in the memory of the Beloved, but also a knowing joke about art’s value. Love gets the music for nothing; everyone else has to bring silver.