CORS is the browser's way of asking a server, before letting JavaScript read its response, whether it has agreed to be read by this origin. Most CORS errors are misunderstandings of who is enforcing what.
In plain language
In security, this is one of the pieces a system uses to keep the wrong people out and the right people in. CORS is the browser's way of asking a server, before letting JavaScript read its response, whether it has agreed to be read by this origin. Most CORS errors are misunderstandings of who is enforcing what. If you are new to the field, the simplest mental model is this: browser rules for cross-origin http requests. Read it once with that frame in mind, then come back and read it again — that is usually enough for the rest of the entry to make sense.
An everyday picture
Think of CORS as a lock on a door. Boring when it works, suddenly the loudest thing in the room when it doesn't. The goal is for it to stay boring.
Where it shows up
CORS runs in the background of any product that handles login, payment, or private data. It is most visible the moment it fails — someone gets in who shouldn't, or someone is locked out who shouldn't be.
A small example
Imagine the scene above. The role CORS plays is the one its blurb describes — Browser rules for cross-origin HTTP requests. When you log in to a bank without anyone in a café reading your password, ideas like this are doing the protective work.
Common misunderstanding
One line to take with you
CORS is a quiet promise. Keep the promise small, write it down, and check it works.