Store Product Properties submenu item allows you to add various custom properties for the product.
Store Product Properties - Add new
You can add new property with the help of "Add New" button.
Store Product Properties - Settings
Within the "Add New Property" block you can specify property name and set various values (for example, size, color etc.). Use "Add Value" button to create as much values as necessary.
Also you can bound each property to the specific product under the Properties tab