API stands for Application Programming Interface, and, generally speaking, it is a software that allows applications to “communicate” with each other. Let’s take a web application as our example. What you see and interact with is the frontend of the web application. On the other hand, data management, processing, and generally, business logic of the application happens on the backend side. Whenever you perform an action on the website (that is, interact with the frontend side), like filling out a form or placing an order, it is the API that actually “communicates” your action to the backend. Once the backend is done processing your “order”, the API brings the result back to you.
A backend API is an interface used by a mobile or web application to communicate with the backend. For example, imagine that you are planning to develop a web application, and then maybe expand your business in the future by developing the same app for Android and iOS. Each one of these apps would require communication with the backend through the API. Moreover, you would be able to re-use the same business logic between various platforms and devices. This will allow you to save time and money, as your future solutions are all built on the same, shared backend architecture.