A little late on this, sorry! But when you're using Curves and click to add a point on a particular curve, there should be little user input boxes for the "Input" and "Output" that will define the point, which in-turn, defines the curve. The curve is a two-dimensional representation of what you're altering in the image. The points are an easy way to define the function, otherwise you'd have to figure out the math to create that particular curve, which can get extremely complicated, depending on the curve. If you'll notice, every curve starts out as a straight, 45 degree line, which is the equivalent to "Input = Output" or in mathematical terms, "x = y".