What is CORS, and what problem does it solve?

CORS (Cross-Origin Resource Sharing) is a browser-enforced mechanism that allows a server to explicitly declare which other origins (scheme+host+port combinations) are permitted to make requests to it and read the response. It relaxes the browser's default Same-Origin Policy in a controlled, server-approved way, enabling legitimate cross-origin use cases like a SPA on one port calling an API on another.