Airdrops reward past on-chain behaviour: using a protocol, bridging, providing liquidity, or holding a particular NFT. Criteria are usually announced after the snapshot to limit gaming, though airdrop-farming persists.
Treat received tokens with care. Unsolicited tokens appearing in your wallet are frequently bait: the "claim" site is a wallet-drainer, and some tokens cannot be sold at all. Never interact with an airdrop you did not expect.
Tax treatment varies by country and can be unfavourable: several jurisdictions treat receipt as income at the market value on that date, which can leave a tax bill larger than the tokens are worth later. Check local rules.
Related: airdrop-farming, sybil-attack, wallet-drainer, testnet