A smart contract is a program that runs on a blockchain and automatically enforces the terms of a contract. For example, if two people agree to a deal where one person will pay the other person a certain amount of money for a good or service, the smart contract will automatically transfer the money to the other person once the terms of the agreement have been met.
Chaincode, on the other hand, is a program that runs on a blockchain and manages the assets on the blockchain. For example, if a group of people are using a blockchain to track who owns a certain asset, like a piece of property, the chaincode would be responsible for updating the ownership records on the blockchain whenever the property is sold to someone else.
In very layman language you can imagine that you and your friend are playing a game where you have to follow a set of rules. A smart contract is like a game master who makes sure that everyone follows the rules and doesn’t cheat. Chaincode, on the other hand, is like a scoreboard that keeps track of who is winning and losing the game.