From a web search:
"That would suggest that the message has left your phone and has not yet arrived on the receiver's phone, still on the receiver's server waiting to be sent to the receiver's phone. Normally this would hint that the phone is off or that there is no signal at their end".
i.e. the message has been successfully been sent as far as the phone company that provides the service to the intended recipient but it can't yet be sent to their phone (together with a request for a delivery receipt) as that phone is turned off or is otherwise unavailable (e.g. because they've run out of charge, they're out of range of a phone signal or they've not paid their phone bill).