We have already seen that the derivative of a function $f$ at
$x=a$ is $$f'(a) = \lim_{x \to a}\frac{f(x)-f(a)}{x-a} = \lim_{h
\to 0} \frac{f(a+h)-f(a)}{h}.$$
We can apply the same idea to get a formula for the derivative of
$f$ at every point:$$f'(\Box) = \lim_{h \to 0}
\frac{f(\Box+h)-f(\Box)}{h},$$ where $\Box$ stands for "anything
you like". Recall that we said you will need to be able to
compute this second form of the derivative, with the limit as
$h\to 0$. That is the only form we can use to compute the
derivative function.
In particular, if we take $x$ as input and associate to it
$f'(x)$ as output, then we have a function.
We see how to compute $f'(x)$ in the video. Fortunately, you
already know how to do this, as you will see.