Airdrop is just giving away coins. There are two types:
1. Registration is required: you need to specify the address to which coins will be sent, sometimes some other data: an e-mail, an social account, etc., there may be requirements for dates, number of messages, etc.
For example, an stellar
airdrop for keybase users is happening now.
2. Tokens are airdropped on the fact of ownership of coins in some blockchain. For example, you have bitcoins/litecoins/dogecoins - you are airdroped with a CLAM. You need to own coins on some date and that's it.
The forum has special topics about airdrops, including in local subforums.