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A trading curb, also known as a circuit breaker, is a point at which a stock market will stop trading for a period of time in response to substantial drops in value. The major securities and futures exchanges have procedures for coordinated cross-market trading halts if a severe market price decline reaches levels that may exhaust market liquidity. These procedures, may halt trading temporarily or, under extreme circumstances, close the markets before the end of normal close of the trading session.

The circuit breakers provide for cross-market trading halts during a severe market decline as measured by a single day decrease in the Dow Jones Industrial Average (DJIA). There are three circuit breaker thresholds—10%, 20%, and 30%—set by the markets at point levels that are calculated at the beginning of each quarter.

You can find much more information about circuit breakers at NYSE website