Meaning:
Literally this term refers to "a circuit breaker," a switching device that interrupts an abnormal or fault current. As a metaphor, it refers to a kind of securities regulatory measure that is used to temporarily halt all trading on an exchange to curb panic-selling. Circuit breakers are used both in broad market indexes as well as for individual securities. The Civil Aviation Administration of China (CAAC) also uses the term to refer to measures to punish airlines that haven't been diligent enough in fighting against COVID-19. According to the rules, inbound international flights will be suspended for a week or several weeks after a certain number of passengers onboard have been tested positive after landing in China.
Example:
A: 你不是说这周去巴黎出差吗？
Nǐ búshì shuō zhèzhōu qù bālí chūchāi ma？
Didn’t you tell me that you would be on a business trip to Paris this week?
B: 我原来订的航班因为熔断取消了，我想想还是决定不去了。
Wǒ yuánlái dìng de hángbān yīnwèi róngduàn qǔxiāo le，wǒ xiǎngxiǎng háishì juédìng búqù le。
The flight I booked was suspended by CAAC after previous passengers tested positive for COVID. I decided to cancel the trip after thinking about it.