These environment variables will be defined for you on process.env. For example, having an environment variable named REACT_APP_NOT_SECRET_CODE will be exposed in your JS as process.env.REACT_APP_NOT_SECRET_CODE.
There is also a built-in environment variable called NODE_ENV. You can read it from process.env.NODE_ENV. When you run npm start, it is always equal to 'development', when you run npm test it is always equal to 'test', and when you run npm run build to make a production bundle, it is always equal to 'production'. You cannot override NODE_ENV manually. This prevents developers from accidentally deploying a slow development build to production.