The ePID identifies a product within eBay catalogs. ePID is returned in the ProductReferenceID field of the FindProducts in the Shopping API.
You can also find the ePID on eBay's Web site. When you search by keyword, search results will return matching products. ePID for the product can be seen once you click into the product.
Yes, you can use the ePID when listing items by passing it in the Item.ProductListingDetails.ProductReferenceID field. It is recommended to use the ePID over the ProductID. See attachment for sample AddItem request.