Okay kiddo, imagine you have a special way of saying things that you really like, but now everyone else starts saying the same thing all the time, that's a cliché! It's like a phrase or an idea that has been used so many times that it has lost its originality and become boring.
For example, if you hear someone say "once upon a time" you might think of fairy tales, right? But if someone uses that phrase in a story over and over, it becomes less interesting and more predictable. People use clichés because they think it's an easy way to express something, but it's important to be creative and use your own special way of saying things. So, try not to rely on clichés too much, kiddo!