Below is a graph that shows the decay of the radioactive isotope barium-141 with starting amount (n). The line tangent to the curve gives the instantaneous rate of change at time (t).
Instantaneous rate of change
This graph shows us the decay of barium-141 given a starting amount (n), and the slope of the tangent line gives us the derivative at time (t) which shows how fast the barium is decaying at any given time. Changing the two variables will show you the varying rates of decay at different starting amounts of Barium and after different amounts of time.