What is a Market-Wide trading halt?

Market-Wide Circuit Breakers

Equity and option exchanges have procedures for coordinated cross-market
trading halts if a severe market price decline reaches levels that may
exhaust market liquidity.These procedures, known as Market-Wide Circuit
Breakers, temporarily halt trading, or under extreme circumstances, close
the markets before normal close of the trading session.These circuit
breakers are determined by a single-day decrease in the S&P 500 Index.

Level 1
S&P 500: 7% Decline
Trigger Time: 9:30 Am – 3:25pm ET
Halt: 15 minutes

Level 2
S&P 500: 13% Decline
Trigger Time: 9:30 Am – 3:25pm ET
Halt: 15 minutes

Level 3
S&P 500: 20% Decline
Trigger Time: Any Time
Halt: Remainder of day

Following a market-wide circuit breaker, exchanges will run reopening
auctions in their respective primary-listed securities.
For more information see: NYSE Market-Wide Circuit Breaker & Nasdaq
Market-Wide Circuit Breaker.