Glossary
API
An API is a socket through which two programs talk to each other directly. Instead of someone copying data by hand from one program into another, the programs send the information themselves, for example orders from your online shop to your invoicing tool. Almost every modern program has an API.
Example: The online shop sends every new order through an API to the invoicing tool, which issues the invoice automatically.
Why it matters to you
Without APIs, people become the cables between programs. With them, data flows on its own.