It’s a quick start method for Mapbox. Upload a picture, and select the colors on your map from the picture.

The very short version is: WordPress will return Json objects from certain endpoints. Here’s a sample:
yourWordPressSite.nu/wp-json/wp/v2/posts
A URL like this will return a Json object with the 10 most recent post. Why don’t you try it in your browser:
https://multimusen.dk/wp-json/wp/v2/posts
But what’s the fuzz all about? Well JavaScript can transmute Json to content on a web page. So now you may program your frontend via JavaScript. This is really good news for Nodejs-developers. But in fact you may develop your web page in any language, that’s able to manipulate Json.
REST API is much more that reading content. You can create, read, update and delete content on the server.
With the REST API WordPress will open the gates to a very interesting future. Perhaps themes as we know them will be obsolete in the near future. Even the Dashboard could be something in the past.
Bang the drums, and hoist the flags! WordPress with REST API is here.