I'm a first-year PhD student as well and usually give a short and simple summary (few sentences) of the project and its goals if someone asks. If you say something like physics, people will automatically ask for details. If you give them a long and complicated title nobody knows what it's about. Usually they want to know what you are doing when you are at work (are you sitting in front of a computer, conducting interviews with people, doing literature research in a library, working in the lab, doing field work outside and so on) or why someone is interested in that topic, so just answer this in a simple way. Not that hard ;) If the person is from a related field you can be more specific.