Smart contracts are simply programs stored on a blockchain that run when predetermined conditions are met. They typically are used to automate the execution of an agreement so that all participants can be immediately certain of the outcome, without any intermediary’s involvement or time loss. 

A contract activated when it receives ETH, or gas. Given the distributed nature of the Ethereum blockchain, the program runs on every node. A feature of the Ethereum blockchain, the main blockchain for DeFi applications.