A lot has been building up inside Montag before he gets onto the subway, and the commercial he hears there is just the final straw, the final push that snaps him over the edge. Before this, he had met Clarisse and really started thinking about things for the first time, only to have her disappear. He had pondered the empty nature of his marriage, Mildred's unhappiness, and the seeming meaningless of everyone's lives. He had burned Mrs. Blake's house, been confronted by Beatty, and was on the brink of going back to his old ways.
While on the subway, he wants to read the book he's taken before he gives it back to Beatty; he thinks that "no phrase must escape me, each line must be memorized". So, he is trying to concentrate on the book, to let it sink in, to memorize it. He is discontented, but no thoughts of full-on rebellion have hit yet. He is planning on returning the book, resuming life. However, the commercial keeps interrupting his thoughts. He can't absorb the book. The commercial keeps pounding out any thought at all, other than its jingle and catch-phrase. He realizes that this is why his society is unhappy but doesn't act: "The people were pounded into submission" by sensory overload. This realization awakens him, and prompts him to go to Faber, where full plans for rebellion are laid.