The numerals 451 represent the temperature at which paper burns. Whether this number is accurate or not, it has become one of the symbols of being a fireman in this society. In Bradbury's dystopia, a fireman is not one who puts out fires but a person whose job it is to burn books.
We learn that Montag has the numbers 451 emblazoned on his helmet. The numbers are also stitched on the sleeve of his uniform and are emblazoned on the kerosene tanks the firemen wear strapped to their shoulders to burn books.
These symbols, along with the salamander and the phoenix, both animals that, in myth, can withstand fire, are part of the regalia of the firemen and are supposed to intimidate ordinary people.