The "4xx" status codes are a group of HTTP response codes that indicate client errors. These errors occur when the request made by the client (usually a web browser) cannot be fulfilled by the server. Common examples include 404 Not Found, which means the requested resource is unavailable, and 403 Forbidden, indicating that access to the resource is denied. These codes help users and developers understand what went wrong during a request. By providing specific error messages, they guide users in troubleshooting issues, such as checking the URL for typos or ensuring they have the necessary permissions to access a resource.