React class: fetch() Post Requests III -destination?

(this questions is related to the ‘festch()Post Request III lesson in -Build Front End Web Applications from Scratch : link https://www.codecademy.com/programs/6a4de1264e49b3e0a75e7e1bf9398ecd/content-items/ff8c8a314c7f5e23e56262ef9f8b6e6a/exercises/fetch-post-requests-iii’)

In the example code, the value of a URL to shorten using Rebrandy is assigned to the property ‘destination’

const data = JSON.stringify({
destination: urlToShorten})

Is ‘destination’ a property of JSON, or JSON.stringify, or is it a something else? I can’t find any reference to ‘destination;’ in any of the reference info online concerning JSON.