RCBO works in a similar way to an RCD measuring the current on the live and neutral conductors. If they are not equal it suggests there is current leakage to earth, which could potentially mean someone is standing on their bed with their finger touching the live conductor of a light as they are attempting to change a light bulb – the RCBO will break the ciruit or trip.
An RCBO also works as an MCB (Minature or Mains Circuit Breaker). In this operation it is designed to interrupt a current overload in a circuit. RCBOs are rated both by the normal current that are expected to carry, and the maximum short-circuit current that they can safely interrupt. A short circuit is an abnormal low-resistance connection between two nodes of an electrical circuit that are meant to be at different voltages, for example if a conductir liquid (water, coffee, tea etc) causes a connection between the live and neutral terminals of a plug or a light switch. This is one of the reasons you will have a pull cord in the bathroom.