A new Altcoin project started with Premine to some percentage, in that the developer uses some percentage to advertise his coin for that he conducts Airdrop which Means he is going to give free coins according to their Bitcointalk forum rank or some other way. In this Airdrop who ever user joins it they get free coins according to their rank and that coins when listed the user can sell immediately or according to their trading strategy and earn free bitcoins. So Airdrop means user gets free coins buy doing some task or filling the google spreedsheet prepared by the coin developer or their bounty manager.