Mean Deviation = 6 + 3 + 3 + 2 + 1 + 2 + 6 + 78 = 308 = 3.75. So, the mean = 9, and the mean deviation = 3.75. It tells us how far, on average, all values are from the middle. In that example the values are, on average, 3.75 away from the middle. For deviation just think distance.
