The term airdrop usually refers to free coins given out as part of a marketing effort to gain traction, increase popularity, or drive up value. You’re essentially being given crypto for free, and it’s dropped into your wallet.
You may receive a new coin as a reward for installing software and trying it out. Or, everyone who holds a certain amount of Ethereum might just be given a few coins of new cryptocurrency for free. Other times everyone who subscribes to a certain podcast on a specific day might be given free crypto. Airdrops can also happen after a hard fork. The owners of the new crypto might decide to give every user an amount of coins equivalent to what they owned on the original blockchain at the time of the fork.