Differential is the gradient of Tangent
Move the slider h to Zero, to change the secant to tangent. The sliders a, b , c and d are all for the change the equation (graph of function) What you are seeing is the quadratic function.
When you move the lisder h to zero observe the ratio Delta(y) over Delta(x), below the gradient of PQ and compare with dy/dx below. you can see this ratio is tending towards the value of dy/dx when h becomes almost zero. This means, dirrential at a point is the gradient of the tangent.