I know that chrome extensions use \'manifest.json`, but here, it is used as something else as well.
contents -
{
\"short_name\": \"React App\",
It's a Web App Manifest that describes your application and it's used by e.g. mobile phones if a shortcut is added to the homescreen.
From MDN (linked above):
The web app manifest provides information about an application (such as name, author, icon, and description) in a JSON text file. The purpose of the manifest is to install web applications to the homescreen of a device, providing users with quicker access and a richer experience.