Okay, imagine you drew a really cool picture and you want to keep it all to yourself so nobody else can see it. But what if other people were trying to draw the exact same picture as you without even knowing you made it first?
That's where a United States defensive publication comes in. It's like showing your picture to everyone and saying "Hey, I made this first!"
In fancy adult talk, it means when someone creates something new like an invention or idea, they can publish it in a document that's available to the public. This way, if anyone else tries to patent or claim they invented the same thing later, the defensive publication proves that the original creator had the idea first and helps protect them from having their idea stolen.
So basically, it's a way for people to protect their ideas from being stolen by sharing them with the world.