Sandwich board

Imagine you have a big piece of cardboard that you wear like a backpack. This cardboard has a picture or a message printed on it. Sometimes people use this kind of cardboard to advertise something, like a sale or a new restaurant. They walk around town wearing the cardboard like a backpack, so everyone can see the picture or the message. This cardboard is called a sandwich board because it looks like two pieces of bread with something in the middle, just like a sandwich.