Research that is successful and leads to publications and recognition is what they are looking for. They like to see concrete measures of achievement, like publications, presentations at conferences, things like that. The specifics are irrelevant, it is more important that you did something and achieved something with it. The higher impact the publication, the better.
Usually people are most successful in research when two things come together, the first it is something they are passionate about or can become passionate about, the second is if they work with an group (advisor) which is working on interesting problems and which publishes impactful and interesting papers. By being actively engaged you are going to achieve more, and by being with a strong group, you are going to be challenged. The key is keep optimistic and keep working hard. Research is a LONG slog with a lot of miss-starts, you have to keep at it, keep trying to creatively solve problems as they arise and keep asking questions and learning.
