From Synfig Studio :: Documentation
This tool draws regions with straight edges. It is deprecated in favor of the Spline Tool which can draw straight edged polygons and also curved shapes.
Here are some example polygons (in case you forgot what they look like):
Note that the edges can cross each other. When they do, the Winding Style Parameter comes into play.
Just click over the different corners of the polygon and finish the creation of it by clicking again over the first vertex.