This means that a TCP RST was received and the connection is now closed. This occurs when a packet is sent from your end of the connection but the other end does not recognize the connection; it will send back a packet with the RST bit set in order to forcibly close the connection.
This can happen if the other side crashes and then comes back up or if it calls close() on the socket while there is data from you in transit, and is an indication to you that some of the data that you previously sent may not have been received.
It is up to you whether that is an error; if the information you were sending was only for the benefit of the remote client then it may not matter that any final data may have been lost. However you should close the socket and free up any other resources associated with the connection.