In everyday terms
Imagine an app that wants to show the weather. It can't compute the forecast itself; it needs to ask a weather service. The path it takes is: "Hey, you have an API. I want today's weather for Istanbul from you."
The service responds: {"city": "Istanbul", "temp": 18, "condition": "cloudy"}. The app receives this data and shows it on screen.
That's what an API is: structured question, structured answer. The entire modern software world runs on top of APIs.
Types
REST API — Most common. URLs name resources (e.g. /users/123) and HTTP verbs (GET, POST, DELETE) describe actions.
GraphQL — More flexible. The client defines exactly the data it needs; multiple sources can be queried in a single request.
SOAP — Older enterprise standard. XML-based, heavy but secure.
API access at Onremo
Onremo's official API is available on Pro plans and above. You can programmatically manage your systems, users, and invoices.
Details: API access guide.