An airdrop distributes tokens to a specified set of addresses, typically as a launch marketing tactic or to reward early users. Recipients are usually selected by snapshot of previous on-chain activity - using a protocol before a certain date, holding specific NFTs, providing liquidity, voting in governance.
Airdrops draw mercenary capital. "Airdrop farming" - performing minimum-viable interactions across dozens of protocols to qualify for future airdrops - has become its own industry. Several recent airdrops have been gamed so heavily that real users were drowned out by farmers operating thousands of wallets.