A shelter in place is the use of a structure and its indoor atmosphere to temporarily separate individuals from a hazardous outdoor atmosphere. This can be becasue of a hazardour material incident, or perhaps a weather related emergency. It entails closing all household doors, windows and vents and taking immediate shelter in a readily accessible location, such as a basement or central medium to small room, and, in the case of a hazardous material incident, trying to make it as airtight as possible by shutting off all ventilation/HVAC systems and extensively sealing the shelter's doors and windows from all outside air contaminants with damp towels, or if available, plastic sheeting and adhesive tape. Diagrams of what sheltering in place entails following a chemical, biological, radiological or nuclear threat, and how long it is advised to be done for, is provided by the FEMA affiliated website ready.gov.
Shelter-in-place effectiveness has been evaluated and experimental results show that proper sealing can make a substantial difference to a normal shelter, finding it to be at least twice as effective against a host of airborne substances when compared against simply staying inside and not implementing the countermeasure, and in most airborne contaminant cases, it is usually much more effective, depending on the particle size of the substance in question.
In practice, depending on the exact situation, everyone within a specific distance of the airborne incident may be ordered to shelter in place or people within a closer range may be ordered to evacuate while everyone else shelters in place to minimize public exposure as much as possible. Sheltering in place is generally only used for a short period of time, typically a few hours.
The phrase has also erroneously been used, instead of the more accurate lockdown, to describe precautions to be taken by the public when violence has occurred or might occur (particularly in shootings) in the area and the perpetrator is believed to still be in the area but not apprehended. The public in the area is advised to carry out all the same tasks as a typical shelter-in-place but without the key step of sealing the shelter up to prevent outside air from circulating indoors, in this scenario people are simply urged to lockdown - stay indoors and "close, lock and stay away from external doors and windows.