CORS (Cross-Origin Resource Sharing) is a security feature implemented by web browsers that blocks web pages from making requests to a different domain than the one that served the web page. This is done to prevent malicious websites from making unauthorized requests to other domains and potentially stealing sensitive information.

When you make a request using the axios library in a React.js application, and the request is for a resource on a different domain than the application, the browser will first send a preflight request (an HTTP OPTIONS request) to the server to check if it is allowed to access the resource. If the server responds with the appropriate CORS headers, the browser will proceed with the actual request, otherwise it will block the request and throw a CORS error.

There are a few reasons why an axios fetch might be giving CORS errors:

1. The server has not been configured to allow CORS: If the server has not been configured to return the appropriate CORS headers, the browser will block the request. To fix this, you will need to configure the server to include the appropriate headers that allow cross-origin requests from your React application’s domain.
2. The server has a firewall blocking the request: Some firewalls may be configured to block cross-origin requests, and therefore, you will have to configure the firewall to allow requests coming from your React application’s domain.
3. Use of a localhost: If you’re developing the front-end and back-end of your application on the same machine and using localhost for both, the server will be considered as a different origin than the client and can lead to CORS error.

To fix the error, you can either use a proxy to make the request, or you can use a browser extension like the Moesif CORS extension to bypass the CORS restrictions while testing.

In summary, CORS errors occur when a browser blocks a request due to a lack of permission to access the resource on a different domain. To fix the error, you must ensure that the server returns the appropriate CORS headers, the firewall is configured to allow requests from your React application’s domain, or you can use a proxy or a browser extension to bypass the CORS restrictions.