sample clothes just means that those clothes were used to put on mini fashion shows for vendors looking to have that particular line of clothing in their stores. Before the clothing line hits the stores, designers show their garments to buyers for different stores (Macy's, Bloomingdales, Nordstrom etc etc) so the buyers can see the line beforehand and if they like it they will have it sold in their stores. The clothes used by the models is slightly worn so they can't sell it new so they have sample sales to still make money off the clothes.
Sample sales are also used to sell left over merchendise from last season's collection. Because of these two reasons you get designer clothes at super low prices.