At the end of Act 1 Linda shows that she is still puzzled about the argument between Biff and Willy and why Biff called Willy "a fake". Obviously, she does not know about the affair that Willy had and that plagues him so much. Linda asks Willy what Biff has against him, but Willy says he is too tired to continue, and then promises to ask for work in New York.
Linda's speech in defence of Willy is certainly memorable and persuasive, as it turns Willy into a dignified victim. However, Biff is unable to reveal his father's affair, which is what makes Willy such "a fake" in his eyes. It is this that troubles Linda, as she suspects that something is being unsaid and this "something" causes her concern and worry.