You put it in a broker slot and then place it in your house, then people can come and buy things in your house and don't have to pay the broker commision.
As the name suggests you can only use it for jewelry. There are other displays for food, weapons, etc as well as general pupose salesmans crates.
In order for people to be able to buy directly from a sales display you have to allow visitors (automaticaly set when you place the display) and have paid your rent. The items are still available through the broker.