The term "quantity supplied" refers to the amount of available product. For instance, if a farmer provides 100 bushels of corn, then this is the quantity supplied of corn.
On the other side, the term "quantity demanded" refers to the amount of items people are willing and able to pay for. Both components must be present. If someone is willing to pay for something, but not able to, then this isn't part of the demand equation. Likewise, if they have the money, but don't want the product, then it doesn't make sense to include this as part of demand either.