Mean is the average. So, if you had the numbers 2, 4, and 6, the average would be 4 (you add all of the numbers together, then divide by the number of numbers, in this case 3).

Mean Deviation is a bit more complicated. I will first give an example to try to explain it.

So, if we used the same numbers as before (2, 4, and 6), you would first find the mean or average.
   2+4+6=12, 12 divided by 3 = 4. The mean or average is 4.

Next, you would find out how far away each number is from the average.
   2 is 2 away from 4.
   4 is 0 away from 4.
   6 is 2 away from 4.
   (Tip: Use a number line)

Next, you would find the mean or average of the distances.
   2+0+2= 4...(Since there is a 0, it would only count as two numbers I think). 4 divided by 2=2.

   The mean deviation is 2.

So, basically the mean deviation is the mean of the distances of the numbers from the mean (it doesn't make sense, I know...)

I hope this helped!