The network topology IOTA is designed around is a mesh network. In order to emulate that topology over the classical internet, full nodes are configured to reject any requests from neighbors that weren't mutually tethered with it.
It is possible that this has come from a neighbor with a changing IP address, an old neighbor that is still trying to connect, or a node simply prodding for traffic.
This message logs the event that it has received a packet from an unknown neighbor, and is rejecting that traffic.