Airdrops  are nothing but promotional techniques used by companies to increase their prospect base. Airdrop is a free distribution of tokens/coins . A true airdrop will not require you to do anything, except putting  an Ethereum address  and your basic details like name , email etc  to receive the tokens but in some cases  you are supposed to follow the companies pages on telegram, twitter, Facebook etc .

to Sum up in air drop you earn free tokens/coins .