what is it and how is it used generally

Basically a shunt trip CB is...
A device (CB) that can be "shut off" from a remote location.

The CB has a coil or solenoid installed that will shut the CB "off" when power is applied to the coil or solenoid. THis option is usually factory installed, but is a field installable item on some large frame (amperage) CB's.

The control voltage can range from 12 volts to 480 volts normally.

There are numerous uses for ST CB's, computer/data rooms, gas stations, elevators, cooking equipment, etc.

John

Remember that a shunt-trip accessory for a circuit breaker requires a (momentary) source of power to operate. An "undervoltage-release" accessory for a circuit breaker requires removal of power to trip.