Merism is a big word that grown-ups use to talk about the way we can describe things by naming their different parts. Let's say you have a pizza with pepperoni, cheese, and mushrooms. Merism would help you describe that pizza by telling you about all of the individual things that make it up.
Think of merism as a superhero that breaks things down into its parts so that we can better understand it. For example, we can describe a car by listing its parts like wheels, engine, seats, and steering wheel. By using merism, we can have a better understanding of the car and what makes it work.
So, even though merism may sound like a big and complicated word, it's really just a way to talk about things by breaking them down into their smaller parts.