It’s an easy way to earn free token by upcoming ICO’s. Many ICO’s run airdrops to increase their awareness and build a big community. If you perform an airdrop you will get the coins to your wallet when the event is over. Another kind of airdrops is a holder airdrop. For example Bitcoin Cash is a fork of Bitcoin. This means at a specific time they do a snapshot of the Bitcoin blockchain and than they get the same amount of Bitcoin Cash like you hold Bitcoins.