An airdrop is when free tokens of a particular cryptocurrency are given out to the community (read: early adopters). The purpose is to gain an early community of users who can get excited about your token/project and evangelize it for you.

Some examples of crypto airdrops in the past are listed here. No harm in getting some free tokens, but stay vigilant to prevent getting scammed. Never give out your passwords or private keys for anything, to anyone, for any reason (even if they promise BIG RETURNS at low risk). Also, before you start evangelizing a new token that you just got for free, think about whether you actually believe in the project (would you put your own money into it?) or are you just partaking in a multi-level marketing scheme.