The variant availability determines if the variant is available for purchase. You must first enable the Product availability feature under Configuration > Product. Once enabled, you'll be able to add your own avalability rules. The Storefront comes with several predefined rules such as allowing a product to be purchased based on user location or security role.
You can also use XSL transform to determine whether this item is available for sale. The expected output should return "true" to indicate this variant is available for sale under the input conditions, otherwise "false" if disallowed. The Storefront comes with several pre-defined rules that you can simply modify the values without needing to know XSL. To learn more about XSL, please see the XSL Transform section.