A questioner is the person who is asking a question. You meet the word most in a public setting: a talk, a class, a news show, a meeting. One person speaks, then a questioner in the room (or on the phone, or in the chat) gets a turn to ask. It names the role, not the question itself. It sounds a little more formal than “the person who asked” — hosts and news writers like it (“our next questioner…”). It is a neutral word, not rude and not a joke. Though actually, the same spelling can also mean the person doing a long, hard set of questions, as in an interview, but everyday English often says interviewer or just the police / the lawyer instead. In class, if you raise your hand, you are the questioner for that moment.