is the hypothesis function (a classifier like AdaBoost, SVM etc).
if P r[y = h(f (x))] is found to be sufficiently high, it means that the feature f (x) is able to distinguish effectively between the actual language L and the approximate languagẽ L. f (x) captures those features of the language that are complementary to the ones captured by the trigram model and therefore f (x) is a good feature to augment the trigram language model with.