Wire-break detection means that in case of a wire breakage no danger emanates from the control unit.
For this purpose, you use a normally-closed contact for switching off. So in case of a wire-break the PLC gets a signal change logic 1 to logic 0, the same signal change as you would have if you press the stop-button for example. Switching on is also protected by a normally open contact, so that the system cannot be switched on in case of a wire break at the start button.
PLC - Wire break