The white curve is a sine curve (or cosine curve if you have clicked "Cos"). The blue curve is its differential curve (gradient curve). The horizontal (x) axis is in radians.
Play around with the values of the parameters A, B, C and D. These affect the white curve, which in turn changes the blue curve. What kinds of affect do these changes have? By investigating, you should get a good idea of what a gradient curve is.

A challenge! Clicking the "random curve" button will draw a random gradient curve in yellow. Your mission (should you choose to accept it!) is to cover this yellow curve with the blue one (not the white one - that's too easy!). You'll have to think about it! Good luck!