In everyday situations, we often do not directly express what we intend, but instead formulate our utterances in ways which appear more polite to hearers. Compare the utterances Pass me the salt! and Could you pass me the salt? Both are in effect requests, but the first one, phrased as an imperative, has a different connotation than the second, which uses the form of a question. It's obvious to us from experience that Could you pass me the salt is not actually a question about the ability of the addressee to pass the salt, but a prompt to action, and responding to this prompt simply by saying Yes, I could and not acting would not be an appropriate reaction. Could you pass me the salt? has two pragmatic levels. On the surface level it is a question, but underlying this is a request. It therefore qualifies as an indirect speech act, whereas Pass me the salt! is a direct speech act.