Press 'esc' for a help screen explaining the use of the mouse and various keys.
I didn't comment it, but will try to answer specific questions, if posted here.
A spline is defined by nodes. The connection between two nodes is called a segment. Every node has two controls. Control a (the red dot) controls the curve leading to that node. Control b (the green dot) controls the curve leaving that node. That means, every segment is influenced by two controls, control b of the starting node and control a of the ending node.