A parallel transaction pipeline, without changing consensus
ERC-4337 makes smart contract wallets first-class accounts capable of defining their own validation logic, all without requiring a change to Ethereum's core consensus rules. Instead of an EOA signing a normal transaction, a smart account owner signs a UserOperation, a structure that looks similar to a transaction but is explicitly not one. UserOperations don't go into Ethereum's regular mempool, they go into a separate, alternative mempool built specifically for them. Specialized actors called bundlers watch that mempool, collect a batch of UserOperations, and package them into a single real Ethereum transaction that calls a public, canonical smart contract called the EntryPoint, which is what actually executes them on-chain.
The EntryPoint is the piece that makes this trustworthy: it's a single audited, immutable contract shared by every ERC-4337 wallet, and its job is to loop through each UserOperation, call the sender wallet's own validateUserOp function to check whether the operation is properly authorized, and only then execute the requested action. Because validation logic lives inside the wallet contract itself rather than being hardcoded into the protocol, a wallet can define authorization however its designer wants, a single key, multiple keys with a threshold, a biometric-backed signature, a time-locked recovery scheme, anything expressible in contract code.