Hey kiddo, do you know that some words can have only one meaning and can't mean anything else? That's called monosemy.
Let's think of an example, the word "dog". "Dog" can only mean one thing, which is a furry pet with four legs and a wagging tail. It can't mean an apple or a book or anything else.
So, monosemy is a fancy word for a very simple idea that some words have only one meaning and can't mean anything else. It helps us understand and communicate better when we use words that have a clear and specific definition.