An airdrop is a means for a new developer or company to introduce their coin to the market. It is done through distributing little amount of coins to many people who enrolled in the airdrop program. By doing so, it gives more people idea about the project and the coin, plus it urges them to use it in transactions, which may help in increasing the value of the coin. It's basically free coin, but the amount is not that much since its purpose is introduction.