The Buy Box is the area at the top of an Amazon product detail page where customers select Add to Cart or Buy Now; Amazon now calls it the Featured Offer.
Many sellers can offer the same product, but only one offer at a time is featured in the Buy Box. Amazon's algorithm awards it among eligible sellers, weighing price together with factors such as fulfillment method, shipping speed, and seller performance. Amazon notes that Featured Offer prices are commonly at or below the lowest priced alternatives, and it shows sellers competitive prices from outside Amazon for comparison.
A Buy Box is described as broken or suppressed when Amazon features no offer at all and replaces the Add to Cart and Buy Now buttons with a link to see all buying options. Because most shoppers buy through the featured offer, a broken Buy Box can sharply reduce a listing's sales. 📝Amazon Specific SKUs are one tactic brands use to avoid it.
In the image below, the Buy Box of this product has been "broken."