We use the word "demand" when talking about something that is bought and sold. "Demand" is how much people want something. When demand is "high", it means that a lot of people want to buy it. When demand is "low", not many people want it.
This is a term that economists use a lot. It also comes up a lot in business news stories:
Demand for nurses and other health services continues to increase.
Some say that their manufacturing output can't keep up with the demand.