In order to find the mean of a set of numbers, you add all of the numbers together and then divide by the number of the numbers. I'm going to show an example since this is confusing.
1, 2, 4, 6, 5, 3
First, add all the numbers together and you get 21.
Then, you divide 21 by 6, because there are 6 numbers, and you get 3.5
So, 3.5 is the mean.
Hope this helped!