In Ray Bradbury's Fahrenheit 451, this quote by Alexander Pope is significant to the plot. In the society in which Montag lives, people exchange words like a box of cereal across a breakfast table: it is a mindless activity.
Words are like leaves and where they most abound,
Much fruit of sense beneath is rarely found.
Pope's quote says that those who talk a lot will often say little of value. He is generally encouraging people to speak briefly—with purpose, and refrain from speaking just to be heard. In Montag's world, all the masses do is exchange shallow, pointless conversation. People are discouraged from having original thoughts, and encouraged to embrace the government's ideas of acceptable behavior, especially pertaining to books. In Montag's society, words are considered dangerous when coming from a book, but spoken words—empty of meaning or value—are preferred as long as they stick to socially accepted rhetoric.
Conversely, Clarisse is someone who actively pursues original thought. She notices things that the brain-numbed masses do not, such as the dew on the grass in the morning. Her questions are purposeful and meaningful. She is not a model citizen in this regard, and she surprises Montag with her conversation, but he finds her words irresistible, even though he is one that burns books.
Beatty, Montag's boss, quotes several famous writers, including Sir Philip Sidney and Alexander Pope. His entire purpose is to convince Montag that words and ideas are of no value. Ironically, in using Pope's quote, Beatty is making his point: Beatty's babbling is pointless. However, the result is not what he hopes to achieve in Montag. Rather than being discouraged, Montag is more convinced than ever of the importance of books and the ideas conveyed therein. Ultimately, Montag will kill Beatty to protect his own ability to pursue the written word.
The quote argues for substance rather than quantity. Beatty tries to use the quote to support his stance that words are meaningless, however, Montag perceives the true essence of Pope's words: it is not the number of words you utter, but the substance of thought and understanding in those words that will "bear fruit," that will be essential.