柠檬不比心的酸
[níng méng bù bĭ xīn de suān]
Translated loosely as 'lemons are not comparable to the ache in my heart,' implying that heartache surpasses any sour taste lemons represent. It uses a play on words involving 'lemons' (similar sound to 'heart pain') and compares the bitterness felt emotionally against actual lemon acidity.