Mrs Gould probably means, "What do you know of our marriage?" and Decoud
answers with firm discretion. But on a deeper level, the line can be read that
Decoud knows nothing at all -- that he is asserting the blindness of universal
subjectivity. Indeed, that is the method of his argument for Separation here:
he appeals to everyone's divergent subjective ideals (his own, Charles', Emily's,
Holroyd's, even Nostromo's), thus asserting at a fundamental level the primacy
of subjective motive over objective knowledge.