JSON provides a simple representation of the key/value properties of an object:
However, someone else might have chosen to use different names for those properties:
Without needing anyone to change their data schema - apart from a backwards-compatible addition of one line - JSON-LD allows each data publisher to map their local property names to the URLs and data types of a shared ontology.
The mapping from local property names to ontology URLs is provided by a JSON-LD context document, linked from the data using a @context attribute:
A parser can then combine the context document and the actual data to produce linked data: