Airdrop is a free tokens that is being distributed to all participants. Most ICO's are giving tokens through Social Media and Bounty Campaigns but for some are they're giving it right away so the best option is giving it through Airdrop. The process would be sending them the application with your information whatever they asked for and your wallet address, then after a moment you'll get your tokens right away.
Most airdrop has specific requirements so be sure before your application will check if you're qualified.