Scarcity means prominently pointing out the limited amount of the product offered, most often on the specific landing pages used as a part of a ad campaign or promotion effort. Otherwise might be used in regular product pages if the amount of the product is limited by stock and the customer would have to wait additional time for the product to become available again.
Example of uses is the number of seats on the plane on booking pages such as Expedia, items left in stock on Amazon or number of free rooms in a hotel on Booking.com. Scarcity is very effective if it is credible. Attempts to use scarcity on items such as copies of files or other items, by definition, unlimited quantity usually backfire.