This is the frontend part of the Segmentation app exercise. In order for it to work the backend (api) must also be accessible, either locally (at http://localhost:8080 by default) or remotely (in this case you must set the REACT_APP_PRODUCTION_API_URL environment variable).
The backend project can be found here.
In your shell just use the npm start command and the application will start at http://localhost:3000.