Q:

What do the 6 points stand for in Polygon().getPoints().setAll?

I have the following code to create a polygon:
Polygon poly = new Polygon();
poly.getPoints().setAll(14d, 17d, 26d, 17d, 20d, 27d); // What do these mean?

What do these 6 points stand for?

A:

The numbers define a triangle:

Giving the x and y coordinates in order like ((X,Y),(X,Y),(X,Y)).
Might be easier to understand if you wrote the code like this:
poly.getPoints().setAll(
  14d, 17d, 
  26d, 17d,
  20d, 27d);

When in doubt you can always check the documentation.

Creates a polygon, defined by an array of x,y coordinates.

