Amazon Standard Identification Numbers (ASINs) are unique blocks of 10 letters and/or numbers that identify items. ASIN is Amazon’s own SKU they use to manage and organise all products. You can find the ASIN in the URL of a product on Amazon.
The ASIN can be found right after the /dp/ part in the link. In this case it is: B07DNBX1C4.
“Fulfillment Network Stock Keeping Unit” refers to the Amazon's own barcodes used for labeling your products to track them in their warehouse. FNSKU labels need to be placed on every product unit that goes into the FBA warehouse unless you opt to use "manufacturer tracked" FBA, which uses the UPC / EAN barcode.