Web Page Metadata - About Property

What is the about metadata property

The subject matter of the content.

Example

Schema.org

Schema.org - https://schema.org/about

{
  "@context": "http://schema.org/",
  "@type": "Article",
  "name": "Apple announces iPhone SE",
  "description": "New iPhone announced at 11:30 in California.",
  "about": {
    "@type": "Event",
    "name": "Apple's March 21 Announcements"
  },
  "contentReferenceTime": "2016-03-21T11:30:00-07:00"
}