What happened to the stock market last Friday?
$20 billion stock fire sale
Major U.S. entertainment companies $ViacomCBS Inc(VIAC.US)$ and $Discovery, Inc.(DISCA.US)$ both dropped27%, their biggest declines ever. In all, $35 billion was wiped off of a range of bellwether stocks.
The U.S. listings of Chinese tech giants also experience huge moves, with the likes of Chinese online education provider $GSX Techedu(GSX.US)$ falling 42%....

Investors are borrowing huge sums of money to buy stocks. Is that a problem?
The “everything rally” that started in stocks last year has been boosted by investors betting money they have borrowed. That includes both small players and heavyweights like Archegos Capital Management, the investment firm that triggered a mini-meltdown for several companies’ stocks.
As of late February, investors had borrowed a record $814 billion against their portfolios, according to data from the Financial Industry Regulatory Authority, Wall Street’s self-regulatory arm...

What are circuit breakers?
Circuit breakers are measures implemented by exchanges when they consider price movements to be overly volatile. They can take the form of a trading suspension or limiting buying and selling of securities within a specific price range. 
Purpose:
Circuit breakers can be triggered to curb in panic-selling or manic-buying. 
Applicable assets:
The circuit breaker system applies to both individual securities and market indexes. As a result, it applies to options market as well ...