Okay kiddo, so you know how sometimes people have secrets they don't want anyone else to know about, right? Like maybe you have a secret hiding place for your toys or a special trick you can do that you don't want anyone to copy.
Well, sometimes grown-ups have secrets too, but sometimes they're not really that secret. Like maybe lots of people know about it, but no one really talks about it out loud. This is called a "secret de polichinelle" (pronounced: se-kret duh polee-shi-nell).
It's kind of like when all your friends know that you have a crush on someone, but no one ever says it out loud. Everyone just kinda knows, but they don't want to embarrass you or make a big deal out of it. So even though it's not really a secret, it's still sort of a secret because no one talks about it openly.
So a "secret de polichinelle" is like a little secret that everyone knows but no one talks about too much. It's like a little inside joke that only the grown-ups get. Does that make sense?