Here is a 1-paragraph summary of the text: Environment variables are variables that describe an environment and can be used in programming languages to make code more flexible and safe by separating configuration values from code. They can be set on macOS and Linux distributions using the `export` command, on Windows using PowerShell, CMD, or the Graphical User Interface (GUI), and in some situations using .env files. Cloud providers such as Heroku, Azure, and AWS also support setting environment variables. Once set, environment variables can be used to access sensitive information like API keys without hardcoding them into code, making it easier to switch between environments and improve security.