The mean of a dataset is the average of all of them.
The absolute deviation of a particular value in a dataset is how far away it is from the mean. If a set contains the number 7, and the mean is 6, the absolute deviation of the value 7 is |6-7| = |1| = 1
The mean absolute deviation is the mean of all of the absolute deviations of the values in a dataset; it's the mean of the absolute deviations from the original mean.