Why do I see ASINs (9 digit alphanumeric strings) in the Search Term Screen?
ASIN stands for Amazon Standard Identification Number, they are used to refer to a specific product page on Amazon. In the case of books, the ISBN is used in its place, that is why you will see ISBN if you are advertising from a KDP account.
These ASINs show up if the campaigns you are running are bidding on a product page placement. If you are running an automatic campaign Amazon will bid on related ASINs automatically.