The mean average is when you add up a set of numbers and then you divide by how many numbers there are.
Say you have a set of numbers e.g. 24, 7, 18, 6, 15. You add them together, so...
24+7+18+6+15=70
You then divide 70 by how many numbers you have,so...
70 divided by 5=14
And so the mean number is 14