He is talking about the Duhem-Quine thesis, and the idea that even if we accept that Hume's critique of induction is rejected (because inductive, or ampliative reasoning is a valid form of inference) underdetermination still poses problems for theory selection. Quine argues that theories can always be held onto in the face of contradictory evidence if one makes sufficient adjustments in the background assumptions of the theory. This is what he means by ampliative UD, and he is critical of Quine's conclusion that it is an important factor in theory choice.