Did you click the link? Because it's clearly explained

edit Updating to be more technically correct:

> https://twitter.com/RobertFreundLaw/status/16517606508579020...

Basically, a retailer must prove they've actually sold a product at it's base price to a substantial amount of customers, to prove that the reduced/sale price is actually a sale price and not its base price. This prevents customers from believing they're getting a better deal than they are.

> The FTC rule says a former price needs to be “the actual, bona fide price at which the article was offered to the public on a regular basis for a reasonably substantial period of time.

Go to website > see 80% off > sign in to buy > no longer 80% off