Elements of sketch.

Projects Programming
1

I try to understand sketch from examples.
From the very beginning I have:
#include "xxx.h" // this line includes library
next line is like that:
xxx XXX;

Where xxx is the name of the library but what is this?
Is it a declaration? of what?
Is it function?
What exactly this statement means?

In void loop() section XXX is called to read data.

Thx in advance

3

Than you Delta_G.
That means this class must be defined in the library.
In my case, xxx it is the class from the library, and XXX is my naming for that class by what object is called. This is how I understand it now.
Thx again