Light Clients & SPV
Simplified Payment Verification (SPV) is the technique light clients use to confirm a transaction using only block headers and a Merkle proof.
In depth
A light client asks a full node for the branch of hashes connecting a transaction to a block’s Merkle root. If the hashes chain up correctly to a header the client already trusts, the transaction is confirmed to be in that block. It is verification without downloading the whole block.
Why it matters
It is how mobile wallets stay both practical and reasonably trustworthy.