What is the meaning of a 'sound and authentic' narration?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Its meaning as explained by Ibn Hajar, and it is an explanation which As-Sayooti has agreed to, is that if the narration has two chains of narrators or more, then it is sound in one chain of narrators, and authentic in another chain of narrators. However, if it has only one chain of narrators, then the meaning is sound in the view of some scholars, and authentic in the view of some other scholars. 

Allaah Knows best.