Airdrop is defined as the process of freely distributing a new cryptocurrency to people hopefully creating more demand.
When a new cryptocurrency is created, it needs to gain users. One way of doing this through an airdrop. The group issuing the airdrop hopes new users will begin researching and sharing the coin creating more demand.
To start an airdrop, the crypto group will often find people who already have another cryptocurrency and identify them as being equipped with wallets that can accept these airdropped coins.
Their other option is to have people voluntarily complete a form where they will then receive their coins.
NOTE: Beware of any website that asks for your private keys. DO NOT share your private keys with anyone unless you trust them with your money!