Application Binary Interface (ABI)

The ABI is the interface with which you call functions in a contract and get data back.

What is an ABI and why is it needed to interact with contracts? - Ethereum StackExchange, 2016 [2 min read]

This stack overflow post gives some insight into what the ABI is, analogizing it to an API.

Application Binary Interface Specification - Solidity Documentation [10 min read]

This section from the Solidity Docs gives a far more in depth overview of how the ABI is constructed and how it works.