I am really confused what is @id used for in json-ld syntax. Sample from apple.com. What does @id actually represent. Any help would be great?<script type=application/ld+json>{    @context: http://schema.org,    @id: http://www.apple.com/#organization,    @type: Organization,    url: http://www.apple.com/,    logo: https://www.apple.com/ac/structured-data/images/knowledge_graph_logo.png?201608191052,    contactPoint: [        {            @type: ContactPoint,            telephone: +1-800-692-7753,            contactType: sales,            areaServed: [ US ]        }    ],    sameAs: [        http://www.wikidata.org/entity/Q312,        https://www.youtube.com/user/Apple,        https://www.linkedin.com/company/apple    ]}
What is the use of @id in json-ld syntax?
schema.org;json ld