A stockroom is the room at the back of a shop or office where the goods are kept until they are needed out front. When you ask in a shoe shop whether they have your size and the assistant disappears for two minutes, that is where they go. It is a working room, not a nice one — shelves, boxes, a ladder — and customers do not usually go in. The word is smaller and more everyday than 'warehouse': a warehouse is a whole building, often far from the shop, while a stockroom is just the back room. In an office it holds paper, pens, and supplies. You will also see it written as two words, 'stock room'; both are fine.