What are identifiers?Edit
An example of an identifier is a social security number, ISBN for a book, Stock Keeping Unit (SKU), or a bar code number for a product. The core concept is that a number explicitly refers to a distinct thing, type of thing, concept, or individual.
Systems like this have existed before databases. Consider Zip codes or Postal Index Numbers or longitude and latitude as examples of identifiers that refer to an explicit geographic area in different ways. (There other non-geographic examples: license plates, credit card numbers, phone numbers, etc.)
If every point on the globe can be represented as a number, it's not such a stretch to imagine every single thing having a unique identifier associated with it.