Cross-Origin Resource Sharing (CORS) 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 an HTTP request that includes an "Origin" header. The server can then respond with specific headers to indicate whether the request is allowed. If permitted, the browser will allow the resource to be shared; if not, the request will be blocked.