The "401 Unauthorized" status code is an HTTP response indicating that a request has not been applied because it lacks valid authentication credentials. This means that the user must provide proper login information, such as a username and password, to access the requested resource. Without these credentials, the server denies access to protect sensitive information. When a user encounters a "401 Unauthorized" error, it typically means they need to log in or provide the correct credentials. This response is part of the HTTP protocol, which governs how messages are transmitted over the web, ensuring secure access to resources.