A smart contract is an app. Similar to an app that lives on your iPhone, except it can’t be upgraded and it lives on a blockchain, meaning that its execution is decentralized (not controlled by a single entity).
We’ll be interacting with smart contracts throughout the Oil Strike. By interacting I mean reading and writing (affecting) the smart contract’s state. Going back to the iPhone app parallel, an example of affecting state in an iPhone app would be recording a new Contact in the Contacts app.