CORS
CORS is a browser rule that decides whether a page from one website may ask for data from a different website.
In a simple example
Your page at yourshop.com asks another service for prices. If that service has not said yes, the browser blocks the answer.
Why it matters to you
It is a common reason for a mysterious error that says blocked by CORS, even when your code looks right.
Do not confuse with
CORS is a protection in the browser. It is not a bug in your code, and turning it off completely is unsafe.