This idiom means a blessing in disguise.
Biānsài yǒu gè lǎorén tā de mǎ pǎo diū le
边塞Biānsài🔊有个yǒu gè🔊老人lǎorén🔊，他的tā de🔊马mǎ🔊跑丢了pǎo diū le🔊。
Línjū lái ānwèi tā tā shuō zhè wèibì shì huàishì
邻居Línjū🔊来lái🔊安慰ānwèi🔊他tā🔊，他说tā shuō🔊：这zhè🔊未必wèibì🔊是shì🔊坏事huàishì🔊。
Guò le jǐ tiān nà pǐ mǎ jūrán dài huí le yì pǐ Hú rén de jùnmǎ
过了Guò le🔊几天jǐ tiān🔊，那匹nà pǐ🔊马mǎ🔊居然jūrán🔊带回了dài huí le🔊一匹yì pǐ🔊胡人的Hú rén de🔊骏马jùnmǎ🔊。
Érzi qí jùnmǎ shuāi duàn le tuǐ lǎorén yòu shuō zhè wèibì shì huàishì
儿子Érzi🔊骑qí🔊骏马jùnmǎ🔊摔断了shuāi duàn le🔊腿tuǐ🔊，老人lǎorén🔊又说yòu shuō🔊：这zhè🔊未必wèibì🔊是shì🔊坏事huàishì🔊。
Hòulái dǎzhàng jiànquán de qīngnián dōu zhànsǐ le tā de érzi què yīnwèi tuǐ qué bǎozhù le xìngmìng
后来Hòulái🔊打仗dǎzhàng🔊，健全的jiànquán de🔊青年qīngnián🔊都dōu🔊战死了zhànsǐ le🔊，他的tā de🔊儿子érzi🔊却què🔊因为yīnwèi🔊腿瘸tuǐ qué🔊保住了bǎozhù le🔊性命xìngmìng🔊。
English Translation:
An old man on the frontier lost his horse. Neighbors came to comfort him, but he said: "This may not be a bad thing." A few days later, the horse actually brought back a fine horse from the Hu people. His son rode the fine horse and broke his leg, but the old man said again: "This may not be a bad thing." Later, when war came, all the able-bodied young men died in battle, but his son survived because of his lame leg.