An allusion is an indirect reference in a text to a work of literature or art, or an indirect reference to a historical, political or cultural event. Works of literature often allude to earlier literary works or to history to set a tone or trigger a set of associations on the part of the reader. Allusions presuppose a shared cultural context, which is why many allusions, such as to Classical Greek literature, are lost on us today.
The book burnings that Montag engages in as a fireman allude to the Nazi book burnings of the 1930s. These book burnings may seem remote to us, but they would have been within living memory of many early readers of Bradbury's novel. In the book burnings, the Nazis made huge bonfires of literature they deemed offensive and unfit for loyal members of the Reich. These included books by Jewish authors, by communists, and by other writers who challenged National Socialist ideology. These overt and dramatic acts of censorship made a deep negative impression on Americans wedded to First Amendment freedoms of expression. They became a symbol of totalitarian abuse of individual rights and a symbol of repression. Therefore, the mere mention of book burning would have raised in readers' minds the strong idea that the political regime of the novel was repressive and malevolent. This would have encouraged readers to side with Montag in his resistance to the regime.
There are many allusions in the novel "Fahrenheit 451". The allusion in the title is a reference to the temperature at which paper burns. Paper burns when it reaches a temperature of 451 degrees Fahrenheit. This is a reference to the job of the firemen in the novel. Ironically, they burn books full of paper instead of putting out fires.