An airdrop is a process where the developers and the respective team of a token / coin distribute their coins to people, which participate in the distribution. In the most cases you have to do something for the airdrop, e.g. join a telegram channel, retweet something on twitter, follow on facebook, therefore they want to increase the popularity of their project / coin / token. But be careful, there is also spam going around, like they request your private key (never give out your private key) or they want that you download their own wallet (here be careful because could be a virus). So on every airdrop, first turn on your brian, like everytime.

If you have any more questions, just let me know.