What does ABI (Application Binary Interface) mean?

The machine-readable list of a contract's functions and how to call them — the inputs, outputs, and signatures — with none of the implementation logic.

To interact with any contract you need two things: its address and its ABI. Compiling a contract (or an interface) in Remix produces the ABI alongside the bytecode, which is what lets other code call its functions.

Go deeper:

• Application binary interface — Wikipedia — diagrams where the ABI sits versus the API across the compile boundary.