Click and drag nodes to select and move them around. Hit DELETE to remove the selected node. Click nearby a segment to insert a new node on it. Click on a segment to flip its direction. Pan and zoom in the preview pane.
This is a fractal curve generator. You make a connected sequence of line segments, the generator, on the grid to the left. Each of your segments is replaced with a new copy of the whole, again and again. After a few iterations (the fractal level) you get the curve to the right. Each segment in the generator has a direction (the little arrow) which determines how the new copy is oriented.