The Fast-sync option helps to sync the FBA inventories between Amazon and Shopify.
Fast-sync will update the Amazon FBA inventory to Shopify based on the locations configured in the app. For each Amazon region, you need to specify the Shopify location that you want the stock level to match Amazon. As soon as an order is placed on Amazon FBA, the app will update Shopify location with the same stock level as on Amazon.
Note: these locations should be created by you in the Shopify admin and not locations that are inventory apps, dropshipping apps (e.g. Oberlo), third-party logistics services, or custom fulfillment services (eBay fulfillment) etc