This means that the warning tone sounds simultaneously on all networked smoke detectors. If, for example, smoke develops in the living room, it can take a relatively long time until enough smoke has come through the closed bedroom door to trigger the smoke detector there as well, but if the smoke detector in the living room and in the bedroom are networked with each other, you may still be able to extinguish the fire yourself, of course only after alerting the fire brigade and the other residents or neighbours.