Airdrop is a free distribution of coins. You fill out an application, sometimes you need accounts in social networks (follow in Twitter, subscribe in Facebook), subscribe to a telegram group, etc. Each Airdrop has its own rules.

The best airdrop in which I participated was BTW (Bitcoin White). 1500 coins, which at one time cost 0.9 cents! But I participated in hundreds of Airdrops, and profit from each more than $ 1 is already good, since most airdrop do not even pay coins.