In the scene you are referring to in Fahrenheit 451, people are crowded into a subway car. There is an advertisement for Denham's Dentifrice blaring repeated on the loud speaker over and over again. The wording changes a bit each time but the message is clear, Denham's wants the riders to buy their toothpaste. It replays over and over again in Montag's brain until he cannot contain himself and speaks aloud, telling the commercial to "shut up." His reaction stuns the other passengers as they either heard it as background or were caught up in the rhythm of the words. They "were beaten into submission." In other words, they heard the same thing over and over again so much so that they had no reaction to it. They gave their subconscious to the message without even thinking about it. Montag was thinking about other things and refused to let the repetitive words break into his thoughts which he believed were more relevant.