There are two primary different ways in which the demand for a good might change.
The first is if there were an increase in the price of a product/good. If the price were to increase, then less people would be able to afford the good. This would therefore lessen the demand for the good.
The second option is if the person's income were to change. If a person were to make $20 per day and he spent all $20 on four $5.00 cups of coffee, then he would no longer be able to afford four cups of coffee if his income changed to $15 per day. If his income changed to $15, then he would only be able to afford three cups of coffee at $5.00 each. Because the person could no longer purchase the same amount of coffee as he used to be able to, the demand for coffee would decrease.
