The most popular product is determine with a recordset that is based on this selection:
sum(ps3_orderdetails.DetailQuantity)
So whatever item in the ps3_orderdetails table that has the largest value in the DetailQuantity column is what will be displayed.
In simple terms, it is the item that has been sold the most, if that is the same as the featured item it will go to the next item. You could populate this by doing some test transactions with the item you would like to be the most popular.