What is Smart contract?
A smart contract is code deployed to a blockchain that executes automatically when its conditions are met. Once deployed it runs the same way for everyone, and anyone can read it.
In practice
Smart contracts hold funds and enforce rules without an intermediary, which also means a bug is permanent unless the contract was built to be upgradeable. That is why audit history, test coverage, and upgrade authority are the first things experienced builders check. Solidity is the dominant language on EVM chains; Rust is standard on Solana.