It is an emergency protocol that usually prevents people or information from leaving an area. The protocol can usually only be initiated by someone in a position of authority. It can also be used to protect people inside a facility or, for example, a computing system, from a threat or other external event. Of buildings, a drill lockdown usually means that doors leading outside are locked such that no person may enter or exit. A full lockdown usually means that people must stay where they are and may not enter or exit a building or rooms within said building. If people are in a hallway, they should go to the nearest safe, enclosed room.

Thanks to Wikipedia

Preventive lockdown

A preventive lockdown is a preemptive action plan implemented to address an unusual scenario or a weakness in system to preempt any danger to ensure the safety and security of people, organization and system. The focus for preventive actions is to avoid dangers and risks arising from the nonconformances to the normal circumstances, but also commonly includes improvements in efficiency.

Emergency lockdown

Emergency lockdowns are implemented when there is imminent threat to the lives or risk of injury to humans, for example, a school’s emergency lockdown procedures must be kept short and simple to make them easier to use under real life crisis conditions. Simple procedures can be easily taught with periodic lockdown drills instead of lengthy training.

In infectious diseases

During the 2019–20 coronavirus pandemic, the term lockdown was used for actions related to mass quarantines. Lockdowns can limit movements or activities in a community while allowing most organizations to function normally, or limit movements or activities such that only organizations supplying basic needs and services can function normally.