Okay kiddo, a monomial is a fancy word for a math expression that has only one term.
Imagine you have a bag of candies. Each candy is like a term in a math expression. If you only have one candy in your bag, that's like a monomial.
In math, we often use letters to represent numbers. For example, if we say "x + 5" that means we have some number (we don't know what it is yet) plus 5. But if we just say "2x" that means we have two times some number (again, we don't know what it is yet) and that's it, we don't need anything else to describe the expression. That's called a monomial.
So, when you hear the word monomial, just remember it means a math expression with only one term, like having only one candy in your bag.