What is 'cookie stuffing', fraud Phoebe Gates' app is accused of?

Phia, the shopping app co-founded by Bill Gates' daughter Phoebe Gates, has been accused of "cookie stuffing". The fraudulent practice involves placing an affiliate tracking cookie on a user's browser without their knowledge. If the user later makes a purchase, the party that inserted the cookie may wrongfully earn commission, even though it did not actually drive the sale.