Habbo credits has a system which allows you to transfer your Habbo credits into Habbo Furniture so you can trade other Habbo credits to other Habbos. You can purchase these by clicking on the Habbo Exchange tab in the Habbo catalogue.

You can put those credits back into coins onto the catalogue by double clicking the coin in a room. It will bring up a pop-up with two options. One will say Cancel on the left and the right will say Exchange. When you exchange, your credit(s) will be put onto the Habbo catalogue. You can see this on the top right of your in-game screen. When they are on the Habbo catalogue, you can use them to purchase items in the catalogue or from the Marketplace.