Instead of saying "Could I have some water, please?" a visitor says "I am thirsty". The host understands it perfectly and says "Let me get you something to drink. What would you like?" or at a diner, the beautiful blond says "I'm dry" and her date immediately calls the waitress. There are lots of situations when we say something and mean something else but the listener understands perfectly what we mean. No ambiguity as in the examples below, where an answer would be considered a joke. What kind of term describes that?
P1 "Can you pass the salt?" P2 "Yes, of course I can."
P1 "Have you got a pen?" P2 "Yes, I do."
Edit - For clarity - A whole sentence (not a word or phrase) means something but we mean something else, and listener understands what we mean.