An agreed way for two programs to talk and exchange data, such as your app and a payment gateway or shipping service.
API stands for application programming interface. Think of it as a menu between two systems: it lists the requests you can make, the information each request needs and the shape of the response. Your app asks, for example, to create a payment for this amount, and the gateway replies with a payment link and a transaction ID.
Most modern apps are built by connecting services through APIs: payments, shipping, SMS, maps and AI. That saves you from building everything from scratch. Your own app may expose an API too, for its mobile app or partners. Browse the available integrations. Practical tip: protect API keys like passwords and never put them in frontend code or messages. Plan for the external service failing: what does the customer see if the shipping provider is slow to respond?