JSON
JSON is a simple way to write information as labelled items so programs and people can both read it.
In a simple example
A product can be written as name: Chocolate cake, price: 8. The site reads those labels to build the page.
Why it matters to you
Settings, menus and data are often saved as JSON, so you will see it whenever you edit a site's content.
Do not confuse with
JSON is only data. It does not do anything by itself, unlike code.