How to ask dumb questions
Keep asking the dumb questions! It is better to look like a fool, than to be one.

You worry that many speakers are annoyed at the `elementary' questions. Some speakers do it because they are stressed about public talking, and any question upsets them. For some, communication of mathematics is not the aim of the talk; they give it because it is a condition of travelling to the venue, or simply because everyone else gives the talks. Instead of feeling joy at an opportunity to clear the confusion, they might get annoyed at having to do the extra work of explaining some of the background. The annoyance has no long-term effect --- nobody holds grudges for asking dumb questions. To give up actually understanding math for such a petty reason is just not worth it.

There is only one situation in which you should refrain from asking a question. That is when you are representative neither of the actual audience nor of the intended audience. So, if you are graduate student at your department's colloquium, it is OK to ask anything. If you are a graduate student at a seminar in your field, it is OK to ask anything. If you are at a seminar in another field, and there are several other students in your field in the room, again it is OK to ask anything. Only if you a lone outsider at a seminar or a conference that is not in your field, there is a reason not to ask questions.