良药苦口 Good medicine tastes bitter
Explanation
比喻忠言逆耳，虽然难以接受，但对人有益。
This idiom means that sincere advice, although unpleasant to hear, is ultimately beneficial.
Origin Story
从前，有个国王，他非常信任一位正直的大臣。大臣辅佐国王多年，常常为国家大事建言献策。有一天，大臣发现国王沉迷于享乐，不理朝政，便上书劝谏。国王听后非常生气，认为大臣是故意找茬，要将他处死。这时，一位老臣站出来说："陛下，良药苦口利于病，忠言逆耳利于行。大臣的谏言虽然逆耳，但却是为了国家好，望陛下三思。"国王听后细细思量，终于意识到自己的错误，接受了大臣的建议，痛改前非，励精图治，国家最终走向了繁荣昌盛。
Once upon a time, there was a king who greatly trusted an honest minister. The minister had served the king for many years, often offering advice on important matters of state. One day, the minister noticed the king was immersed in pleasure and neglecting his duties. He wrote a letter to advise the king. The king was furious and accused the minister of trying to undermine him and ordered his execution. An old minister stepped forward, "Your Majesty, good medicine tastes bitter, but it cures the disease. Honest words are unpleasant to the ear, but beneficial to conduct. Though the minister's words are unpleasant, they are for the good of the state. Please reconsider." The king reflected and realised his mistake. He accepted the minister's advice, changed his ways, and governed diligently. The country ultimately prospered.
Usage
常用于劝诫他人，说明忠告虽然逆耳，但对人有益。
It's often used to admonish others, explaining that although sincere advice is unpleasant to hear, it is beneficial.
Examples
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良药苦口利于病，忠言逆耳利于行。
liáng yào kǔ kǒu lì yú bìng, zhōng yán nì ěr lì yú xíng
Good medicine tastes bitter, but it cures the disease.
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老师的批评虽然很严厉，但确实是良药苦口啊！
lǎoshī de pīpíng suīrán hěn yánlì, dàn quèshì liáng yào kǔ kǒu a
Although the teacher's criticism was harsh, it was indeed bitter but good medicine!