Prompts
A prompt is a request for additional information given by a moderator to a player whose answer to a question is not ruled sufficiently correct to receive points but has been deemed "close enough" by the writer. Receiving a prompt allows a player an opportunity to give an additional answer. A player may receive multiple successive prompts ("prompt-chaining").
The set of answers which warrant a prompt is enumerated in the answerline alongside the acceptable answers.
The most common reason for a prompt is that a player has answered with a superset of the correct answer. For example, a question on "the second Punic War" would prompt on "the Punic Wars".
Philosophy
Prompts are a recognition that questions are not typically written precisely enough to allow only a single correct answer (or set of answers).