What does "four defenses" mean?

The combination of four defenses refers to the combination of civil air defense, physical defense, technical defense and psychological defense.

Taking four precautions at the same time is a safety management method, including four measures, namely, civil air defense, physical defense, technical defense and psychological defense. Civil air defense refers to improving safety awareness and preventive ability through the management, training and education of personnel; Physical defense refers to the management and control of articles, equipment and buildings to ensure safety; Technical defense refers to preventing and responding to security incidents through scientific and technological means; Psychological defense refers to improving the psychological quality and pressure resistance of personnel through psychological consultation and psychological counseling.