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.