Discounts (%) tells you how much of net revenue was taken off from discounts. The lower the discounts %, the closer your customers are paying to full price for your products. A common strategy for new customers is to offer a "welcome" discount, and subsequent purchases with no discount; thus, you should see a higher discounts % for one-time customers than for repeat customers.
Discounts % = Discounts / (Discounts + Net revenue) (* 100%)
In Shopify, discounts % can be derived from subtotal_price and total_discounts.