It is part of a naming convention popularized by tools like , Vite , Next.js , and Vue CLI . The file is designed to override other environment files without being committed to your version control system (like Git).

To understand this file, you have to look at its name in three parts:

.env.development.local (gitignored): REACT_APP_API_URL=http://localhost:5000 LOCAL_DB_URL=postgres://dev:password@localhost:5432/devdb FEATURE_X=true