The HTTP 401 Error, also known as "Unauthorized," occurs when a user tries to access a resource that requires authentication but fails to provide valid credentials. This error indicates that the server recognizes the request but refuses to fulfill it due to a lack of proper authorization. When a user encounters a 401 Error, it typically means they need to log in or provide a valid username and password. This error is often seen in web applications and APIs, where secure access is necessary to protect sensitive information or resources.