For example, when OPEC increases crude oil production, crude oil's price falls. As the supply curve for gasoline moves right, the intersection point with the gasoline demand curve moves down and to the right. Gasoline's equilibrium price falls and its equilibrium quantity rises. More gasoline is sold, at lower prices. When supply falls, the reverse happens.
Less gasoline is sold, at higher prices.
Demand can go up if people want the product more or if people have more money to spend. Suppose you want to do something like increase the number of people who get vaccinated. In economics, a market has a demand side and a supply side. Marketing in the business sense refers only to the demand side of a market. Marketing is demand management.