Scarcity occurs when there is a limited amount of a certain item. Because of how limited that item is, it attracts more attention and increases demand. A real-world example of a website that uses scarcity is Groupon. This site encourages users to buy items quickly with the sale ends soon warning label. Because of tactics that use a warning saying that sales are ending soon, sales will usually increase to a higher amount fairly quickly.
Sources
blog.hubspot.com
Anthony's Blog 