What does this for JavaScript stand for?

In view of the question of what this is all about, now I would like to collate this information as I personally understand it. Please correct me for any errors.

first Case :
if it is a global function, This is equivalent to the window object .

This prints out the result:

It is obvious from the printed result that this time this is equivalent to window.

Second case:

The printing results are as follows:

The third case:

The printing results are as follows:

Fourth case:

If you use new to instantiate an object, This is not equal to the window object,this instance pointing to function a

The printing results are as follows:

What does this for JavaScript stand for?