The star rating that you will see for each bookseller represents that seller's order fulfillment rate for the previous 90 days. Order fulfillment is the percentage of orders placed with that seller which were filled.
Your order fulfillment rate will appear to customers in the form of stars.
Here's what the stars mean:
★★★★★ 96% to 100%
★★★★ 90% to 95%
★★★ 85% to 89%
★★ 70% to 84%
★ 0% to 69%