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:
vavg=ΔsΔt,
where Δs is the distance traveled and Δt is the time elapsed.
We use the Greek letter Δ to mean "change in". If we start at position
s(t0) at time t0 and end up at position s(t1) at time t1, then
Δs=s(t1)−s(t0),Δt=t1−t0.
The following video goes over these ideas, and compares them to the slope
of a secant line.