Imagine you have a filing cabinet with different folders for different projects. An internal transaction, also called a "message," is like a small note you might send between folders within the same cabinet. It happens on the blockchain when an account (EOA - Externally Owned Account) interacts with a smart contract (a program stored on the blockchain) and results in some Ether (a type of cryptocurrency) being transferred within that same smart contract. So, it's a mini-transaction that stays within a specific area of the blockchain.
