It's free distribution of coins to the community for a small task like joining their telegram group etc sometimes for nothing.
Why an Airdrop?
This is like when a new grocery store open in a market they give a free sample to try. This free sample helps them to introduce to their customers. It is better than advertising on Facebook or Google ( As advertising cost much more ). This is also better than ICO because it doesn't require investment.This is done to ensure early distribution of coins & to have as many people "skin in the game" as possible.
1. Bitcoin Cash gave current Bitcoin holders an equivalent amount of Bitcoin Cash. At the time of the airdrop, if you were holding 0.4 Bitcoin, you were one of the many lucky receivers of 0.4 Bitcoin Cash. 2. Bitcore rewarded every Wallet bellow blockheight 463604 with a Balance bigger than 0.01 BTC, all wallets containing BTC at that point can claim free BTX coins by signing a message from your wallet. After you claim your free coins if your wallet has at least 10 BTX you will receive a 3% bonus payment on a weekly basis. 3. Omise gave away five percent of their OmiseGO cryptocurrency to holders of Ethereum in September 2017
