Okay kiddo, so a broken plural is when certain words in English have a different way of forming their plural version. You know how when we want to have more than one of something, we add an "s" to the end of the word? Like dog becomes dogs? Well, broken plurals don't follow that rule exactly.
Some words change their vowels or add a new one to create their plural form. For example, the word "man" becomes "men" and "tooth" becomes "teeth." Other broken plurals don't have any noticeable changes at all, but they still have a different way of becoming plural. For example, the word "sheep" doesn't change at all in the plural form.
So basically, broken plurals are a way of showing that some words have different rules for becoming plural than just adding an "s" to the end. It can be a little bit tricky to remember which words are broken plurals, but with practice, you'll get the hang of it!