A Cubic Bezier is defined by a start point, an end point two control points which describe the slope of the curve at the beginning of the line and at the end of it.
In SVG a Cubic Bezier can be defined as a path, or as part of a path. In this case the path definition contains a Move part defining the starting point and a Cubic Curve part defining the two control points and the end point.
React and SVG is used for display, Redux is used for state handling, and RxJS is handling the drag & drop.
The drag & drop handling of this pen is described in the following article: http://codepen.io/HunorMarton/post/handling-complex-mouse-and-touch-events-with-rxjs