A lead-in (or leadin) is the first clue of a tossup, and thus, per pyramidality, also the most difficult clue in a tossup. A good lead-in contains a pronoun early in the sentence that informs the player of what type of answer is specifically being asked for (i.e. a book, an element, a concept, etc.).
Some lead-ins may contain information that is more tangentially related to the subject at hand. Examples include literary criticism about a novel (rather than descriptions of the plot or characters), opinions of historians about a given historical event (rather than specific facts describing said event), or literature about works of art (rather than descriptions of the work of art itself). Quizbowl does not specifically value such "tangential" lead-ins over any other lead-ins.
Though tangential academic information is acceptable in lead-ins to academic tossups, pop culture clues should be avoided for lead-ins (unless the question is intentionally a mixed pop culture/academic question).
The term "lead-in" also refers to the introduction (or intro) of a bonus, which occurs before the first bonus part proper. Bonus lead-ins typically either identify the theme of the bonus, or give a brief clue (possibly an interesting or little-known fact) about the answer to the first bonus part. In modern quizbowl theory, these are recognized as the only two types of bonus lead-ins.
What to avoid
Bonus lead-ins that are overly long, or have (usually terrible) jokes or other funn content, are often criticized and should be avoided.