Used in auctions of multiple items, winning bidders pay the amount they actually bid. The multiple bidders will most likely pay different amounts for the same item in a Yankee auction.
In the follow example there are three identical Fenton vases. Joe bids $25, John bids $19, James bids $26, Jill bids $17. The winning bidders James, Joe and John all paying their respective amounts, so the vases will sell for $19 - $26.
Compare this to a Dutch Auction, where all winning bidders will pay the same amount.