In short, a smart contract is a special program that is stored in the blockchain and executes there.
The usage of smart contracts is a major breakthrough in the blockchain since the inception of Bitcoin. They are secure and scalable. Almost any business logic can now be transferred to the blockchain without worrying about its fault tolerance.
In Free TON, a smart contract is hidden behind each address. Yes, wallets are also special contracts.
The most interesting is the DeBots and DePools contracts. The first ones are analogs of telegram bots but on the blockchain. You can use them in the ton.surf application. DePools are staking contracts, on which you can deposit funds and receive a certain income for validation. You can read more in our article about staking.
NFT tokens are also smart contracts.
If you are interested in writing your own smart contract, then read our series of articles about it.