this sentence means to deeply miss and love someone, and to express feelings through red leaves and music. No matter where you are, you only have each other in your heart.

This is a poetic sentence, expressing deep thoughts. "Red leaves last the longest", which is a symbol of autumn, falling in the autumn wind, symbolizing the sadness of parting and the long-term "one song for missing". This sentence means expressing the feelings of missing through music, pinning the feelings in your heart on music and letting it pass on to people far away.

"There is no one else in sight, but you are everywhere". This sentence expresses my deep yearning for someone. No matter where I am, I see the shadow of that person and only have the other person in my heart.