A smart contract is a program stored and executed on a blockchain. Once deployed, it runs exactly as written — there's no customer support team that can override it, and (on most networks) its code can't be quietly changed afterward. This is what powers decentralised exchanges, lending platforms, NFT marketplaces, and most of DeFi.
"Smart contract" is a slightly misleading name: it's not legally smart and not a contract in the traditional sense, it's simply code that executes automatically. That also means bugs or oversights in the code are real risks — a contract will faithfully execute a mistake just as reliably as it executes the intended logic, which is why independent audits matter.