A sound bite is an audiolinguistic and social communications phenomenon whose nature become realized in the late 20th century, helped by people such as Marshall McLuhan. It is characterized by a short, meme-like phrase or sentence that deftly captures the essence of what the speaker is trying to say. Such key moments in dialogue (or monologue) stand out better in the audience's memory and thus become the "taste" that best represents the entire "meal" of the larger message or conversation. Sound bites are a natural consequence of people placing ever greater emphasis on summarizing ever-increasing amounts of information in their lives.
News media in particular cherish sound bites. Reporters agree that the best news footage contains at least one sound bite. Politicians in turn have learned (along with their speechwriters) to put greater effort into delivering the perfect sound bite. Originality is not necessary but highly valued. Sound bites are useful to help guide footage editors focus on parts of dialogue that help advance the overall message.
Not everyone enjoys hearing sound bites. They tend to sound best when delivered unplanned, and the reverse is often true -- the planned sound bite can easily ring forced and cast doubt as to the speaker's integrity.
Classic examples of sound bites include Ronald Reagan's demand that "Mr. Gorbachev, this wall must come down!" in reference to the increasing social pressure to remove the Berlin Wall. In this context, the well delivered sound bite serves as a cultural icon that others are likely to know about. Another memorable one is "Houston, Tranquility Base here. The Eagle has landed." In one succint phrase, the entire Apollo Moon program was culminated and everything the US had worked so hard for was suddenly made real. The quality of the best sound bite is that "the message hits home".