Environment variable
An environment variable is a private setting kept outside your code, such as a key or a web address that changes between computers.
In a simple example
Your app reads AI_KEY from the settings of the host, so the real key is never written inside the code files.
Why it matters to you
It keeps secrets out of the code you share, and lets you use different values for testing and for the live site.
Do not confuse with
It is not a variable inside the code. It is set on the machine or the host, and the code only reads it.