The HTTP 401 Unauthorized status code indicates that a request made to a server requires user authentication, but the provided credentials are either missing or invalid. This response is typically seen when a user tries to access a resource that is protected and requires login credentials, such as a private webpage or an API endpoint. When a server responds with a 401 status, it often includes a WWW-Authenticate header, which specifies how the client can authenticate itself. This helps users or applications understand what type of credentials are needed to gain access to the requested resource, ensuring secure communication over the internet.