Could you please tell me what is API?

Answer 1

In computer programming, an application programming interface (API) is a set of routines, protocols, and tools for building software and applications. much more is here https://en.wikipedia.org/wiki/Application_programming_interface

Answer 2

application program interface.
library include in this.

Answer 3

HTML5 File API aspect provides an API for representing file objects in web applications and programmatic selection and accessing their data. In addition, this specification defines objects to be used within threaded web applications for the synchronous reading of files.

Answer 4

I wouldn't stress too much about it right now.. You will probably learn about it in the Javascript tutorial. Maybe also a little bit in css