柠檬少年空凉城
[níng méng shăo nián kōng liáng chéng]
It stands for 'The lemon youth in the hollow-cool city.' It could be a way of saying a lively (like a lemon) young soul contrasting against a cold, impersonal city or urban environment. Possibly inspired by some form of literary work where a character feels detached in a bustling city despite his vivacious personality.