CORS stands for Cross-Origin Resource Sharing, which is a mechanism that allows restricted resources on a web page to be accessed from another domain outside the domain from which the first resource was served. The same-origin security policy forbids certain "cross-domain" requests, such as Ajax requests, by default. CORS defines a way in which a browser and server can interact to determine whether it is safe to allow the cross-origin request. It allows for more freedom and functionality than purely same-origin requests, but is more secure than simply allowing all cross-origin requests.

Here are some key points to understand about CORS:

• CORS is an HTTP-header based mechanism that allows a server to indicate any origins (domain, scheme, or port) other than its own from which a browser should permit requests.
• CORS supports secure cross-origin requests and data transfers between browsers and servers.
• CORS enables a web programmer to use regular XMLHttpRequest, which supports better error handling than JSONP.
• CORS is not a protection against cross-origin attacks such as cross-site request forgery (CSRF) .
• CORS vulnerabilities arise primarily as misconfigurations, so prevention is a configuration problem.

CORS is useful because complex applications often reference third-party APIs and resources in their client-side code. For example, an application may use a browser to pull videos from a video platform API, use fonts from a public font library, or display weather data from a national weather database. CORS allows the client browser to check with the third-party servers if the request is authorized before any data transfers.