CORS (Cross-Origin Resource Sharing) errors occur when a web application on one domain tries to access resources on another domain without proper permissions. Browsers enforce the "same-origin policy" for security, meaning that web pages can only make requests to their own origin unless the target server explicitly allows cross-origin requests.

For example, if your web app hosted on mydomain.com tries to fetch data from api.otherdomain.com, the browser checks if api.otherdomain.com allows this by looking at the response headers. The server can allow cross-origin requests by setting the Access-Control-Allow-Origin header to permit specific origins or to allow all origins with a wildcard (*). If this header is missing or doesn’t match the request's origin, the browser will block the request and raise a CORS error.

CORS errors are particularly common in applications that interact with third-party APIs or microservices from various domains. They can be resolved by configuring the server to include the appropriate headers or by using proxies.

Understanding and handling CORS is essential if you’re looking to learn full stack web development, as it affects both client and server-side interactions.