Answer
Horizonal axis is X-axis and vertical is Y-axis.
You can see that as $h$ approaches to $0$ our graph becomes good approximation.
It explains the the formula of derivative for the function $f(x)=x^{3}$ in the graphical form.
Work Step by Step
As the mode of h getting smaller and smaller(i.e., approaches to $0$ from either side) from $2$ to $0.2$ our graph becomes good approximation to $f(x)=3x^{2}$.
If you know the formula of derivative, it's the same.
If $k(x)=x^{3}$,
$\frac{dk}{dx}=3{x}^{2}$ ...(1)
And in the form of limit it becomes,
$\frac{dk}{dx}=\lim\limits_{h \to 0}\frac{\left(x+h\right)^{3}- x^{3}}{h}$ ...(2)
Compare both the equations,
$3{x}^{2}=\lim\limits_{h \to 0}\frac{\left(x+h\right)^{3}- x^{3}}{h}$
You can see that ultimately both are the same.
So, It's just visualization of our differentiation formula.
