Mean absolute deviation.
Let's break it down into its components.
Mean - Average
Absolute - absolute value - sign doesn't matter - if negative, make positive
Deviation - difference from the mean

Let's do an example. Population 3 7 4 2. Total 16. Mean (Average) 16/4 = 4.

Deviations
3 - 4 = -1 absolute value 1
7 - 4 = 3
4 - 4 = 0
2 - 4 = -2 absolute value 2

Sum of these deviations 1 + 3 + 0 + 2 = 6
Mean (average) of these deviations is 6 / 4 = 1.5.

Good luck,

Paul