An environment variable is a named setting that a computer makes available to a running program. It is like a note handed to an app when it starts, telling it which mode or service address to use.
For example, a website might read a setting to choose between a test service and its live service. Developers can change the setting without rewriting the app's main code.
Environment variables are sometimes used to provide private credentials, but they are not automatically secret or protected. Keep passwords and keys out of public files, logs, and screenshots, and use the hosting platform's secure settings for sensitive values.