JSON-LD stands for JavaScript Object Notation for Linked Data. On a web page it appears as a script tag of type application/ld+json holding a JSON object that describes the page using Schema.org terms, for example its type, headline, author and dates.
Its main advantage is that it sits separately from the visible HTML, so you can add or change structured data without editing the page layout, and it is easy for code to generate from your database fields. That suits frameworks where pages are built from data.
Keep these habits: generate it from the same source as the visible content so they cannot drift apart, escape text properly so the JSON stays valid, use absolute URLs, and test output with a validator. A single syntax error can make the whole block unreadable.
Related: Article Schema, Organization Schema, FAQPage Schema, Technical SEO.