In the social and physical sciences, doctoral level research solves a problem that has never been solved before. In engineering and applied social sciences such as counseling psychology, it extends the frontiers of how to construct something.
The difference between an undergraduate and a masters degree is that an undergraduate degree is sort of a technician's degree. It is just enough information to be able to claim a very basic level of expertise in some field. A person with a master's degree is able to teach undergraduate courses and does not usually need supervision. That person is a master in their field. Their research shows that they have sufficient technical expertise that they should generally be able to operate alone.
At a doctoral level, the goal is to change the understanding of some phenomenon by showing that what we thought we knew was wrong. It is important to understand that while undergraduate degrees are earned, graduate degrees are awarded. Passing all of your doctoral coursework is not sufficient to be awarded a degree.
A committee is formed with a few people from outside the department to be sure that the degree is being validly processed and to provide insight from other disciplines. That committee reviews the work. My doctoral dissertation was around 400 pages with appendices and solved a fundamental problem in multiple disciplines. John Nash's dissertation was four pages. It included the title page and the references. They were such an amazing four pages that he won the Nobel prize for it. Mine was so long because I had to nail down all possible opposing points of view as it was a controversial topic. I had to not only show I was correct, but that the various existing methodologies could not be correct. As the number of tools thrown at the problem was very large, I had a very large number of groups to answer both theoretically and empirically.
You are to be the world expert on your specific topic. You are teaching your professors how the world really works. When you pass them, then you are a doctor. I have a friend who is showing how certain chemical processes work on another planet. Just think for a second how hard that research is to get approved and to do.
You have to identify an original topic and convince your advisor that it can be done in a reasonable amount of time. Do note though that if you do not complete your research, then you do not graduate. Unlike an undergraduate degree, nobody knows if or when a doctoral student will graduate. You can be the best student in the program and not graduate. The role of the advisor is to do their best to prevent sure failures. This is why you often work in the lab on projects they are already working on. They have a pretty good idea that it will work out eventually.
Of course, they don't know how it will work out or if it will work out, but their familiarity with the topic gives them a feel for the probability of failure. You need to choose a program based on who is doing the type of research you are interested in. If you have a chance to go to the number one program in the country, but the research is not what you are interested in, or the number eighty-three program in the country, but the research is what you want to do, then pick number eighty-three. That is your number one.
It doesn't have to be groundbreaking, but it does need to be unsolved.
