What is a Market-Wide Trading Halt?

U.S. equity and options exchanges follow procedures known as Market-Wide Circuit Breakers (MWCBs), which temporarily halt trading across markets if there is a severe, single-day decline in the S&P 500 Index. These halts are designed to prevent panic selling, protect market liquidity, and allow time for information to be absorbed. In extreme cases, the markets may close for the remainder of the day.

The detail:
Circuit breaker levels:

• Level 1: 7% decline in the S&P 500 between 09:30 and 15:25 ET → 15-minute halt
• Level 2: 13% decline in the S&P 500 between 09:30 and 15:25 ET → 15-minute halt
• Level 3: 20% decline in the S&P 500 at any time during the trading session → markets close for the rest of the day

After a circuit breaker halt, exchanges reopen trading with auctions in their respective primary-listed securities.

https://www.nyse.com/markets/nyse/trading-info