I think that the meaning of this is that Clarisse's fundamental question of whether or not Montag is happy has impacted him to a great extent. The idea of "Are you happy?" was a question that was asked in a unique manner, but was one that lingered in the psyche of Montag. It struck at him, haunted him and began the process of reexamination of his life and in what he believed. It also represented the basic idea that Montag's examination took place on public and private levels. The fact that Montag looks at a wall in his house, in his own realm of the private, and begins the process of questioning his own sense of purpose and identity in it is representative of the process of maturation and questioning that Montag undergoes throughout the novel. It is the basis of his own advancement as a character. It is with this moment, the idea of seeing a wall and projecting on it his own newly discovered moments of doubt and inquiry that reflects that Clarisse's question has impacted him on a profound and significant level. From this point on, he is simply unable to "go back" to how things were.