You will find a few threads about this on the Altcoins Discussion section of the forum. Basically and airdrop is a free distribution of tokens. A true airdrop will not require you to do anything, except perhaps registering an Ethereum address to receive the tokens (some airdrops may be new coins or based on NEO, WAVES or other platforms).

This is something that many companies do to have a wider user base and raise awareness of their project.