CORS stands for Cross-Origin Resource Sharing. It’s a security mechanism implemented in web browsers that restricts web pages from making requests to a different domain than the one that served the initial webpage. This prevents malicious scripts from stealing data from other websites.

In the context of AWS, CORS becomes important when you want to allow client-side web applications loaded on one domain to access resources stored in an S3 bucket on a different domain (which is often the case). By enabling CORS on your S3 bucket, you define which origins (domains, ports, protocols) are allowed to make requests to the bucket and under what conditions (allowed methods, headers). This allows authorized access to your S3 resources while maintaining security.