What is public/manifest.json file in create-react-app?

manifest.json is the json file that is mostly used to describe our app On mobile phones

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.