6. a small strip of coloured cloth signifying membership of an order or award of military decoration, prize, or other distinction. 7. a small, usually looped, strip of coloured cloth worn to signify support for a charity or cause: a red AIDS ribbon.
The books burned were Jewish, Marxist, Bolshevik, or other disreputable or non-German books. These included the works of Heinrich Heine, Sigmund Freud, Thomas Mann, Erich Maria Remarque, Albert Einstein, Walter Benjamin, Karl Marx, Jack London and Margaret Sanger.
The number 451 refers to the temperature (in Fahrenheit) at which the books burn when the firemen burn them for the good of humanity. Written in the early years of the Cold War, the novel is a critique of what Bradbury saw as an increasingly dysfunctional American society.