They are the storage expansion that are required to upgrade your Steam Table,Vegetable Racks and Refrigerator to increase their storage space. 

Upgrading the Steam Table helps you store more cooked dishes; likewise, upgrading the Vegetable Rack and Refrigerator helps you store more vegetables that you've grown in your Backyard and ingredients you buy from Store.
You can check the number of expansion items available by following the steps below:
 1. Tap the Vegetable Rack/Steam Table/Refrigerator 
2. Tap the 'Upgrade' button 
3. A dialog is put up displaying the expansion items you need to upgrade. 

For more detailed information, please scroll down to the image below to see where the expansion items can be viewed in Star Chef.