An airdrop, in the cryptocurrency space, is a form of marketing that involves sending small amounts of coins or tokens to wallet addresses as a reward for a completed action by the recipient. The marketing method is usually employed by start-ups in order to raise awareness and increase ownership of the new cryptocurrency to help bootstrap a new project.
Airdrops are usually promoted on social media and recipients need to have a digital wallet that is able to receive the cryptocurrency in question. The recipients of the airdrop receive the tokens for free but they usually have to perform a small promotional service such as making a social media post about the cryptocurrency in return.
Bear in mind that a legitimate airdrop never requires capital investment, it should only be for promotional purposes. However, companies sending small amounts of cryptocurrency to unsuspecting recipients is what is known as a dusting attack, which is why crypto wallet holders should always remain vigilant.