Stock is what you sell in your Commercial levels to earn credits. Each store carries three different items as stock, each of which beomes available as each bitizen is hired. You must place orders for the stock items before they can be sold; the time required for an order to be fulfilled depends on the total skill level of the bitizens you have in the shop. Once an order is ready, the yellow stock icon will apear in the lower left corner. Selecting the icon will take you to the shop where the stock is ready to be sold.
If a level is completely out of stock (and is not currently being stocked), a darker yellow exclamation point will show.