HTTP Status Code 501 signifies Not Implemented, indicating that the server does not support the functionality required to fulfill the request. When a server responds with HTTP Status Code 501, it means that the server either does not recognize the request method or lacks the capability to fulfill the request. This status code often appears when a client sends a request using a method that the server cannot process, such as PUT or DELETE, which might not be implemented on that server. In such cases, the server is informing the client that the functionality is not available, and the client may need to adjust their request accordingly or choose a different server that supports the desired operation.