JSON-LD is short for JavaScript Object Notation for Linked Data. It’s essentially valid JSON, but with a particular type of approach. The idea is that by imposing some constraints on the JSON syntax, unambiguous links can be drawn between datasets spanning very large domains through the use of common a common Namespace, accessed via URI.
This is how things like Google’s Knowledge Graph works. How Alexa and Siri get answers to their questions. How Wolfram Alpha operates (to some extent).