Market-wide circuit breakers in simple words are pre-determined values in percentage figures that prompt an automatic check when a runaway move occurs in any security or index on either direction. These market-wide circuit breakers are triggered by the movement of the NIFTY 50 or BSE SENSEX, whichever is breached first.
When triggered, they cause a coordinated trading halt in all equity and equity derivative markets across the nation.
Check out this page on NSE for more information.