Most of the time this is the following question asked after asking how to earn bitcoin without investing. Most of the people who doesn't want toinvest goes to either airdrop or bounty hunting. As per people with time they choose bounty hunting and for people who want's it the easier way they will go with airdrop. Anyway to answer your question Airdrop is a free coin offer to people who will sign up for the coin and it's upcoming ICO. It is the easiest way to earn but a low chance earn guarantee.