Environment variables are key-value pairs used to configure applications, storing sensitive information like API keys and database credentials outside the codebase for security and flexibility.

Environment variables play a vital role in configuring applications by providing a way to store settings and sensitive information outside the codebase. They are key-value pairs that can be accessed by applications at runtime, allowing developers to customize application behavior without hardcoding values in the source code. This approach enhances security, as sensitive information, such as API keys, database credentials, and configuration settings, can be stored securely and not exposed in version control systems. By using environment variables, developers can easily manage different configurations for development, testing, and production environments, ensuring that applications behave correctly in each context. For example, a developer might set different database connection strings for local development and production environments. Tools like dotenv can be used to manage environment variables in development, loading them from a .env file. By leveraging environment variables, developers can create more secure, flexible, and maintainable applications that can adapt to various deployment scenarios without code changes.