This is two contactors KM1, KM2 control motor forward and reverse circuit.
KM1, KM2 if the same time action will make the main circuit serious short circuit, resulting in accidents.
So the normally closed contact of KM1 is connected in series in the coil circuit of KM2, and the normally closed contact of KM2 is connected in series in the coil circuit of KM1. Once the KM1 power positive, KM2 can not get power, from the objective to prevent short-circuit accidents.