Physical security is the employment of walls, fences, doors, alarms, cameras, locks, keys, etc., to protect and secure valuable things and information. For an organization or business, physical security involves many other issues such key management (locks, doors, and keys), safety, and the responsibilities of individuals within the group.
Physical security is an important component of information security.
If you have an air-gapped computer, physical security becomes even more important. For some, physical security is paramount because of the threat of surrepticious entry.