Circuit breakers temporarily halt trading when share prices hit predefined levels. They were put in place in the US markets with the approval of the Securities and Exchange Commission (SEC) to give investors some time to understand what's happening when there are large and sudden changes in share prices. Sometimes the halt can be 5 minutes, but when the volatility is larger, the markets can close for the day. 

For individual companies, a circuit breaker can be triggered if the share price increases or decreases rapidly (e.g. a company's share price goes up or down by 15% in five minutes).

Broader market circuit breakers only kick in when there is a sudden drop (e.g. if the S&P 500 drops 7% in one market day).
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Circuit breakers are designed to avoid panic-selling - when a circuit breaker kicks in, it means the markets are operating as they are supposed to. They were first put in place after the 1987 share market crash and then updated in 2010 after a 'flash crash' (rapid drop in the market that bounced back quickly), that the existing circuit breakers failed to stop.

Because the US share markets are largely electronic, drops in share prices can quickly become more extreme when they trigger automated buys and sells on mass. By halting trading, those automated triggers have a chance to be tweaked and every day investors have some time to understand what's happening and calm their emotions.

On Monday 9 March 2020 EST, a circuit breaker was triggered when the S&P 500 dropped by 7%  four minutes after the markets opened. The 15 minute break appeared to work as it was designed. After trading resumed, the S&P 500 only dipped a little more, and closed 7.6% lower than the opening price.
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In the US, circuit breakers have three tiers. 

Level 1 halt (7% drop)

Level 2 halt (13% drop)

Level 3 halt (20% drop)