A Merkle tree hashes pairs of items upward until one root hash commits to the whole set. To prove item X is in it, you only need the few sibling hashes along X's path — a handful of hashes instead of the entire list. That is how a contract can verify 'this wallet is on the airdrop list' without storing ten thousand addresses.
It is the standard trick wherever on-chain storage is expensive: whitelists, state verification, light clients, certificate transparency. The pattern to remember — commit a small hash on-chain, keep the big data anywhere — is the same one behind most 'verify off-chain data on-chain' designs.
Related terms
On-chain
Recorded on the blockchain itself — permanent, public, verifiable by anyone — as opposed to off-chain data in someone's database.
Smart contract
A program deployed to a blockchain that runs exactly as written, that anyone can call, and that usually cannot be changed afterwards.
SPL token
Solana's token standard — the shared program every fungible and non-fungible token on the chain uses, so wallets and apps speak one format.
The bench this belongs to
BlockchainSolidity on Ethereum, Polygon, Base and Arbitrum. Anchor programs on Solana. Written with tests first, because a redeploy is not a hotfix once value is on the line.
