少年不知心底蓝少女不知内心涩
[shăo nián bù zhī xīn dĭ lán shăo nǚ bù zhī nèi xīn sè]
It means 'boys don't understand the hidden depth while girls do not understand the inner pain'. It reflects that when young, people are still immature and do not fully comprehend deeper emotions or the hardships in their heart yet.