What exactly is api?

API (application program

Application programming interface is some predefined functions, whose purpose is to provide applications and developers with the ability to access a set of routines based on a certain software or hardware, without accessing the source code or knowing the details of the internal working mechanism.

Generally speaking, API is a windows sockets for others to use. In C, it is usually a function, and in Java, it also includes classes/interfaces. These interfaces, systems or interface providers all provide implementations, and you can use them directly. You don't need to rewrite a bunch of code to realize the functions of these interfaces, just call the method name of API to realize the functions you want.

The API provider provides this set of interfaces for others to use. Of course, if people want to use it correctly, they need relevant help documents to tell users what functions this interface can achieve and how to use it. This is the role of API help documentation.

The API is not only available in Java, but also in C, C++, js and other programming languages. The API can be provided not only by the system, but also by the third party. You can write a set of functions (interfaces) to realize some functions and then give them to others. For people who use your function, the function you provide them is the API of your interface.

Text help documents just tell users how to use the relevant API.

I don't know if I made myself clear.