An airdrop is a way to promote a cryptocurrency and happens 1 of two ways that I know of. The first way is to rewards your crypto to those that hold a certain crypto. So, for example I think it was Stellar Lumens gave so many Stellar for each Bitcoin you owned. The other way to enter an airdrop( feel like it is more of a bounty) but ICO's have been doing airdrops to promote their coin before release and it usually involves signing up to twiiter, fb, telegram, etc. In return you are rewarded with free coin. Hope that helps