It is the standard deviation without squaring it.
Basically it tells you how spread out from the mean your group of numbers is.
Here's how to do it:
Step 1: find the mean (add all numbers together, divide by how many numbers there are)
Step 2: subtract each number in the list from this mean
Step 3: take the absolute value of each number (so they are all positive numbers)
Step 4: find the mean of these new numbers
That's the Mean Absolute Deviation!