Okay, imagine you have a toy car. You know how to make it go forward and backward by pushing certain buttons. But what if there was a secret button on the bottom that no one told you about? That would be an undocumented feature.
In computer programs or apps, sometimes there are secret things you can do that no one tells you about. These things might not be useful or they might even be a mistake, but some people like to figure them out and share them with others.
It's like finding a secret room in your house that no one else knew about. It's not supposed to be there, but it's kind of cool to know about it anyway. However, just like with your toy car, sometimes these undocumented features can cause problems if you accidentally push the wrong button. So it's important to be careful and not rely on them too much.