A research question is a statement that your thesis strives to answer. It is essentially the core focus of your thesis. Everything you are doing or writing about in your thesis is directed toward answering your research question.
Your research question guides your entire research project from the literature review, the methodology, right through to the data collection, analysis and discussion - these all must be responding to your research question. Your thesis examiners will be very carefully looking to see that all of your chapters do, in fact, address what you say you are going to do.
It is normal for your research question to essentially refine over the course of your research particularly between Milestone 1 and 2. Once you get into your research and analysis, and the more you start to know about your topic, the better you can articulate your research aims and objectives. So don’t worry if you feel that it’s not quite right at this stage - you are not committed to the research questions you come up with for the research proposal and can refine or update them after Milestone 1.
There are differences between disciplines - for example the sciences tend to have very clearly defined research questions right from the start, while humanities based research questions may be more fluid and emerge out of your investigation. Some methodologies such as grounded theory actually require you to revise the research questions as you go through, so changing your questions is baked into the methodology.
It’s also common to have more than one research question, in fact up to 5 is quite usual. Often there will be a main question and some sub-questions. If you are doing a thesis by publication, each of your papers will likely have its own research question, but you would still have an overarching research question or aim for the project as a whole.
The most important thing is that, in the final piece of work that you submit, the research questions (as well as your aims and objectives) match what your thesis actually does.
One of the most common sticking points early on is having a topic, but not yet a question. A topic is a broad area of interest - for example, “international student wellbeing” or “renewable energy adoption.” A research question narrows that topic down to something specific and answerable.
For example:
If you can describe your project in a single broad phrase but struggle to state it as a question with a question mark at the end, that’s a sign you’re still at the topic stage. Keep asking yourself “what specifically do I want to find out about this?” until you arrive at something you could actually go and investigate.