Merkle Trees
A Merkle tree hashes transactions in pairs, level by level, until one "root" hash summarises them all.
In depth
Instead of storing every transaction hash in the block header, a block stores just the Merkle root. To prove a single transaction is in a block, you only need a handful of hashes along its branch rather than the whole block. This is what lets lightweight wallets verify payments without downloading the full chain.
Why it matters
Merkle trees keep blockchains verifiable at scale and make light wallets possible.