Windows Environment Variables are smart variables that expand to the correct path on the user machine. For example, when you want to place a file on user desktop in your code, you can not use the absolute path to the desktop since you don't know the name of the user. So instead you would use %HOMEPATH%/Desktop. The part with the % in its name is the Windows environment variable that points to the user home directory. For example this path, on my machine, expands to C:\\Users\\eirannejad\\Desktop

↓ Find a list of these variables and their expanded values here

Complete list of environment variables on Windows 10 * Pureinfotech