An airdrop is a marketing practice whereby a cryptocurrency is given away, usually for free, to individuals who help promote that cryptocurrency.
Often, Airdrops will have specific social marketing requirements to receive payment, such as joining a particular network, retweeting a marketing message or inviting new users to the project.
Since the purpose of an airdrop is usually to spread the word about a certain cryptocurrency coin or exchange, it’s been workable. Furthermore, the new token-holders are incentivized towards the success of the project due to owning a certain number of coins themselves.
The philosophy behind airdrops is simple: more coins lead to more interest and exposure, which also increases the trading volume of a particular coin.