The CSV Headers are labels for your data and they should be the first line in your CSV file.
Example
In the below CSV content the line line sku, quantity, purchaseCost, purchaseDate, receiptDate, id defines the headers.
sku, quantity, purchaseCost, purchaseDate, receiptDate, id
sk22,2,30000,12/23/2017,2/15/2019, 1
sk33,2,30000,2/23/2017,4/15/2019, 2