Okay, so you know how you are in school right now and you have different grades like 1st grade, 2nd grade, and so on? Well, when you finish all your grades in school and become a grown-up, you might decide to go to more school to learn even more things about a special topic like medicine, law or engineering.
When you go to this extra school after finishing college, it's called graduate school. Usually, graduate schools are programs that are more difficult and require you to do a lot of reading, writing long papers, and studying for tests.
When you finish graduate school, you become a master at your chosen topic! But sometimes, you might want to keep learning even more, like a super master. That's when you might decide to spend one more year learning even more about your special topic.
This extra year is called your Postgraduate Year (PGY), and it's like another level of school after all of the others. During this time, you will get hands-on experience doing what you learned in graduate school and get even better at it. For example, if you want to be a doctor, you'll get to work in a hospital and learn how to take care of people.
So, in summary, a postgraduate year is like an extra year of school where you learn even more about your special topic in a hands-on way, after you have finished graduate school.