This quote is from the first part of the book, in which Montag begins to reflect upon the true nature of what he does for the first time. Montag works as a fireman, who in his dystopian society is a person who burns books. Here, he begins to realize that in just a few seconds, he has destroyed what a person probably spent a long time, maybe their whole life, writing. This is unsettling to him and he is clearly growing increasingly curious about what might be written in the books.
He is also growing increasingly frustrated that people around him do not share in his curiosity. His wife Mildred listens to his thoughts and tells him to "let me alone." But this bothers him. "We need to not be let alone," he tells her. "We need to be really bothered once in a while. How long is it since you were really bothered?" Montag is clearly bothered that people just burn books without considering what is written in them. Now that he has started to wonder more about them and realize how pointless his work is, he will never be the same.
The exact page this quote is on will depend on what version of the text you are reading. For instance, in the 50th Anniversary Edition of the book, this quote is on page 51, but in the 60th Anniversary Edition, it is on page 49. It is also on page 49 in the attached pdf of the text.