Final answer:
A vertical demand curve indicates that the quantity demanded of a product does not change with price, representing a perfectly inelastic demand, typically for essential goods with no close substitutes.
Step-by-step explanation:
A vertical demand curve signifies that the product has a perfectly inelastic demand. This means that regardless of the change in price, the quantity demanded remains constant. Consumers will purchase the same amount of the product irrespective of any price increases or decreases. This scenario is generally observed in the case of essential goods, where consumers cannot easily find substitutes, such as life-saving medicines or basic utilities like electricity. As a result, demand for the product is not sensitive to price changes.