This list is an array of Item Specifics, which are category-specific fields that the seller added to describe the listing. The names of these fields are different for items in different categories, so they're returned in a generic name/value structure.
For example, Item Specifics for a car might include a field like Make=Toyota (where Make is returned in Name, and Toyota is returned in Value) and Model=Prius (where Model is returned in Name, and Prius is returned in Value).
In multi-variation listings, the same name cannot appear in both the VariationSpecifics node and in the ItemSpecifics node.
For FindProducts, this can also be an Item Specific that is defined for a product. That is, Item Specifics can be returned both for items and products in FindProducts.
See the Field Index to learn which calls use NameValueList.