Unicity distance is like having a secret code that nobody else can understand. Like when you and your friends use your own secret language to talk about things without others understanding.
But sometimes, if someone knows what kind of secret language you are using, they might be able to figure out what you are saying even if they don't know the exact words. That's where unicity distance comes in.
Unicity distance is the minimum amount of secret code needed to make sure that nobody else can figure out what you are saying. It's kind of like having a big enough secret language that nobody else can break it no matter how hard they try.
So if you want to keep your secrets safe, you need to use a secret code that is long enough that nobody else can figure it out. And the longer your secret code is, the more secure your secrets will be. That's what unicity distance is all about - making sure that your secrets stay safe and sound.