A warehouse is a place where a company stores things they are going to sell. If you order a book online, it is almost certainly shipped from a warehouse to your house.
When a business has to store a lot of items, they often use a warehouse, or a large building, to keep them safe and organized until they're sold or made into a finished product. You can use the word warehouse as a verb, too: "I need to warehouse all these roller skates, because they're not selling as fast as I'd hoped." Warehouse combines ware, or "manufactured goods," and house. In other words, it's a place to house your wares.