To get the instantaneous veloctity at a particular time $t=a$,
we compute the average velocity between time $a$ and $a + \Delta t$,
and then take a limit as $\Delta t \to 0$.
In the animations below, \(a=3\), the \(x\) coordinate of point \(P\); and \(a+\Delta t\) is the \(x\)-coordinate of point \(Q\).
The following video shows how this
is done, and relates it to the slope of the line tangent to $y=s(t)$
at time $t=a$. In both cases we get a limit:
$$\lim_{\Delta t \to 0} \frac{s(a+\Delta t)-s(a)}{\Delta t}.$$
This can also be written as:
$$\lim_{x \to a} \frac{s(x)-s(a)}{x-a},$$
where $x=a+\Delta t$.
This quantity (if the limit exists) is called the derivative of $s(t)$ at time $t=a$.