Introduction
API (Application Programming Interface) is a way for two or more computer programs to communicate with each other. It is a decoupled interface that program(s) can interact with.
Basics
For web developers, an API typically refers to a server exposed to different endpoints (URL paths) to which we can send network requests. APIs act as a layer between our client and the server. An example is a weather API from which we can get weather data.
APIs are not limited to web development. Gaming consoles have a native API exposed that you can interact with when developing games. Mobile devices have their own native APIs exposed for when you’re developing on those devices.
As a front-end developer, you are typically not concerned with creating APIs, only interacting with them (“consuming” them) so that you can receive or send data.
Additional resources
rlogical Techsoft: What is REST API vs. Web API (vs SOAP API)?