When executing the command
npm run build at the root directory of a React app you are preparing the application to push a testing or production environment.
What the command does is minify (strip the whitespace) and bundles all your JS and CSS code each into single file so your browser only has to load one JS and CSS file to render the React application.
These bundled files are located at the root of the applications directory under a
build folder. Often that build folder is created using
mkdirp which makes it a private directory so to view it on the command line run
ls -a from your the root of your application.