Airdrop refers to the FREE coins (free bitcoins/free crypto) given during the Intial Coin Offerings - ICO of the crypto companies in the hodlers' wallet in order to publicize their tokens.
Airdrops involve cryptocurrency projects (or blockchain companies) rewarding users free coins in their existing wallets, subject to meeting certain conditions. Its become quite the trend as as it essentially translates into free money for the users receiving them (provided the coins being given out are valued by the market).