An airdrop is a marketing practice established in the crypto world, in which a token or cryptocurrency (usually startup) team sends their cryptocurrency or token to a big number of users who have signed up to receive their tokens.
Airdrops are a common marketing strategy employed by many new crypto and token projects. The purpose of an airdorp is order to gain traction, fans and an initial user-base for a token or crypto project; the hope is that this initial user-base will then in turn, via word of mouth and other means, invite their friends, who will invite their friends...
This practice is arguably effective as the new token holders who have received the tokens or crypto via an airdrop have an incentive to spread the word and help the project succeed, as in that case their own tokens will be worth more on the market.
Airdrops are sometimes completely free, and all one has to do is sign up. Other times, users have to complete a task in order to receive coins or tokens - this is where airdrops overlap with crypto bounties.