CORS, or Cross-Origin Resource Sharing, is a security feature implemented in web browsers that allows or restricts web pages from making requests to a different domain than the one that served the web page. This is important because it helps prevent malicious websites from accessing sensitive data from another site without permission. When a web application tries to fetch resources from a different origin, the browser sends a preflight request to check if the target server allows such requests. If the server responds with the appropriate headers, the browser will proceed with the request, ensuring safe and controlled data sharing between different origins.