API, in plain words

An API lets one program ask another program for something. You do not walk into the kitchen. You ask the waiter for item 12 and a plate comes back. Most you will meet are REST APIs: you ask for things by web address and the verb says what you want, GET to read and POST to create.

The answer comes back with a status code and a body. 200 means it worked, 401 means "I do not know who you are", 404 means nothing is at that address, 429 means you are asking too fast. The body is usually JSON, plain text written as labelled pairs. A webhook is the other way round: instead of asking every few minutes, the other system tells you when something changes.