Airdrop is basically a "marketing" strategy in crypto where you get free coins of another, often newer crypto, if you hold a certain crypto.

Most recent example I can think of is if you held NXT in bittrex, you were airdropped free IGNIS tokens at a rate of 0.5 per 1 NXT.

The scam I presented here claimed that for each ETH you held, you would get a certain number of EOS for free.

Hope that answers the question!