PhD stands for Doctor of Philosophy. It's a research project where you study something that gets you really excited, for about three years! It is normally the first time a scientist gets to choose what they are researching and what experiments they are going to do.
PhDs are not just for scientists. People do PhDs in all kinds of things! For example you can do a PhD in nursing, music, history or art!
A supervisor is a person who has lots of experience in what you are studying. They are there to help and guide you through your experiments, and offer advice when you get a bit confused or unsure of what to do next.