To keep it simple, we may think of the buyers as consumers. The demand relationship expresses that willingness and ability for the whole range of prices. To say that a person has a demand for a particular product is to say that the person has money with which to buy and is willing to exchange the money for the good. People will not demand what they do not want or need, but a want or a need unbacked by purchasing power is not a demand.
Supply also means willingness to sell.
But it is not. Need without purchasing power will not create effective demand in the marketplace.