Full Idea
The objects of a theory are not properly describable as the things named by the singular terms; they are the values, rather, of the variables of quantification. ..So a referentially opaque context is one that cannot properly be quantified into.
Gist of Idea
Objects are the values of variables, so a referentially opaque context cannot be quantified into
Source
Willard Quine (Three Grades of Modal Involvement [1953], p.174)
Book Reference
Quine,Willard: 'Ways of Paradox and other essays' [Harvard 1976], p.174
A Reaction
The point being that you cannot accurately pick out the objects in the domain