Polyline is a collection of connected lines. The points that determine the lines are listed in the "points" attribute. Each point has X and Y coordinates.
If you move the colored dots in the "Design" area, you can notice how coordinates of each point change in the "Code" area.
The value of fill is set to "none" in this example. If you uncheck that, the fill attribute will be removed from the code, and will take the default value - black.