Okay kiddo, a kohen is a special type of Jewish person who comes from a family that has been chosen to be priests. They are descendants of Aaron, who was the brother of Moses.
Now, being a kohen comes with some special privileges and responsibilities. For example, they are the only ones who are allowed to perform certain religious ceremonies in the temple.
But being a kohen also means they have to follow some strict rules. For example, they can't go near dead bodies and they can only marry certain people.
Even though there aren't really any temples anymore for the kohanim to do their religious ceremonies, people who are kohanim still keep following those special rules because it's an important part of their identity and heritage.