What does interpret the value of the average rate of change mean?

This is my problem and my math book isn't giving me any information about what this question means..
"The graph shows the population P in a small industrial city from 1950-2000. The variable X represents the number of years since 1950."
Graph below points are on X = 20,25,40,50 ; Y= 40,50,40,20
50-
40-
30-
20-
10-
0 ____________________________
--------------10 20 30 40 50
(a) Determine the intervals on which the function P is increasing and on which it is decreasing.
My answer= Increasing [0,20] [20,25] ; Decreasing [25,40] [40,50]
(b) What was the average rate of change of P between X=20 and X=40 ?
My answer = 40 - 40 / 40-20 = 0/20 = 0
(c) Interpret the value of the average rate of change that you found in part (b)
** This is where I am lost. I'm not sure what the question is actually asking, I've done some searching on the internet about the value of the average rate of change, although I think I'm either finding the wrong thing, or not quite catching the concept. Intuitively I consider the fluctuation between the two intervals, but as the past has shown me, intuition and math don't necessarily make good partners ;X

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