Before we start talking about instantaneous rate of change,
let's talk about average rate of change. A simple example is
average velocity. If you drive 180 miles in 3 hours, then your average
speed is 60 mph. We get this by dividing the distance traveled by the time:
$$ v_{avg} = \frac{\Delta s}{\Delta t},$$
where $\Delta s$ is the distance traveled and $\Delta t$ is the time elapsed.
We use the Greek letter $\Delta$ to mean "change in". If we start at position
$s(t_0)$ at time $t_0$ and end up at position $s(t_1)$ at time $t_1$, then
\begin{eqnarray*}
\Delta s & = & s(t_1) - s(t_0), \cr
\Delta t & = & t_1 - t_0.
\end{eqnarray*}
The following video goes over these ideas, and compares them to the slope
of a secant line.