How to answer interview question what did you do undergrad dissertation (for phd interview)
There are several ways to approach this. What your interviewers want to get from that question is how you present something that you (should) know well.

You could just tell them in a very dry manner the goal of the work and what your results were. That will only show them that you did some research and you probably know what you did.

What is probably more impressive is to tell them what kinds of problems you solved and what tools you used to do it (programming, cooperation with other people, trial and error, etc.). From that they will get a glimpse of your work ethics and how excited you are about your work.