Allusions are references to other works of literature or to historical documents, people, or events within a novel, play, or poem. Fahrenheit 451 is filled with allusions.
The novel, for instance, alludes to three playwrights: Shakespeare, Shaw, and Pirandello, when Faber, plotting subversion with Montag, says:
Oh, there are many actors alone who haven't acted Pirandello or Shaw or Shakespeare for years because their plays are too aware of the world. We could use their anger.
Bradbury alludes to Shakespeare again when Beatty speaks contemptuously to Montag, quoting from the play Julius Caesar:
There is no terror, Cassius, in your threats, for I am arm'd so strong in honesty that they pass by me as an idle wind, which I respect not.
In that quote, Brutus is responding to Cassius's statement: "Do not presume upon my love." Beatty communicates here that he doesn't pay any attention to or respect anything Montag has to say.
At the end of the novel, Granger, while talking to Montag, refers to Thoreau's Walden, a book which challenged conformity, and to the English philosopher Bertrand Russell.
Granger also alludes to the Magna Carta and the US Constitution, both important documents safeguarding the rights and freedoms of ordinary people. Granger says:
So long as the vast population doesn't wander about quoting the Magna Charta and the Constitution, it's all right.
In other words, the state doesn't want people asserting their freedom.
The many allusions to literature are Bradbury's way of showing there is an important world beyond the clowns on the televisions screens, and a way to illustrate that books are worth reading.