Short answer
API stands for Application Programming Interface — a way for two systems to talk to each other. When your webshop sends an order to Fortnox, it does so via the Fortnox API. When Stripe reports a payout, it does so via the Stripe API.
For ecommerce businesses, APIs are the foundation of all system integration.
How does an API work?
Think of an API as a waiter. You (system A) place an order, the waiter (API) takes it to the kitchen (system B), and comes back with the response.
Do I need to code?
If you use an integration platform like Junipeer — no. The platform handles all API calls behind the scenes.