An airdrop is when a project sends tokens for free to the wallets of a group of users, with nothing required in return, usually as a marketing strategy or to reward those who already used the protocol.
How eligibility is usually decided
Many DeFi projects have rewarded, with an airdrop, the wallets that interacted with the protocol before launching their own token (so-called airdrop "farmers"), favoring early users over those who show up only after the distribution becomes known.
Risks to watch out for
Not all airdrops are legitimate: there are scams that ask you to connect your wallet to a fake website or sign a malicious transaction under the promise of claiming tokens, which can drain your wallet. It's always worth applying DYOR before interacting with an unfamiliar airdrop.
Taxation
In Spain, receiving free tokens through an airdrop can be treated as a capital gain subject to IRPF (Spain's personal income tax), based on their market value at the time you receive them, even though you paid nothing for them.