JSON-LD stands for JavaScript Object Notation for Linked Data. It is a W3C standard for expressing linked data in JSON; the current version, JSON-LD 1.1, has been a W3C Recommendation since 16 July 2020. On websites you use JSON-LD to add structured data: a separate block in the HTML that describes what is on the page.
Why does it matter?
Google also supports Microdata and RDFa, but recommends JSON-LD because it is the easiest to maintain. You do not need to scatter attributes through your HTML; the block stands apart from the layout.
Example
<script type="application/ld+json">
{
"@context": "https://schema.org",
"@type": "Organization",
"name": "De Spaak Bike Shop",
"url": "https://www.example.com"
}
</script>@context states which vocabulary you use, here Schema.org. @type states what you are describing. With @id you can give an item a fixed identifier, so other blocks can refer to it.
Common mistake
One comma too many or too few. A single syntax error makes the whole block unreadable, and nothing on the page shows it. A second mistake: two plugins or apps that each add their own block with conflicting data.
How do you check it?
Search the page source for application/ld+json and test the code with Google’s Rich Results Test. Atlas, the research engine developed by Outseen, checks whether JSON-LD it finds is readable and, for supported types such as product and breadcrumb, looks at the fields. Which types an online shop needs is covered in Which structured data does an online shop need?