Hi there. I was in the same position to you just over a year ago. I did a BSc and MSc but was still petrified at doing interviews and alot of the reasearch went over my head. However, be brave. Interviewers are not there to trick you, they just want you to show you have a basic understanding of the project and perhaps a few initial ideas. To be honest if you really dont understand the area, then maybe that paticular PhD is not for you. Just pick a project that resembles past reaserch/lectures you enjoyed (usually that is the easiest to understand) For me, biochemistry and i just dont get along!!!