The demand relationship is the relationship between Price ($) and the Quantity Demanded (how much or how many are wanted).

For each price, the demand relationship indicates the quantity the buyers want to purchase. The higher the price, the lower the quantity demanded.

Consumers demand less of an item when prices are high.

There must be willingness on both sides. The supplier must be willing to sell and the buyer willing to buy, at a price agreed to by both. 

Example: When pink Doodads cost $15 each, consumers are willing and able to purchase them at that price.

When the price of pink Doodads rises to $25, fewer consumers are willing and able to purchase them, and the demand falls.