In Mathematics, the mean is the average of a data set, calculated by adding all numbers of the set together, then dividing by the number of elements in the set:
data set: 5, 4, 3, 2, 1 (five elements in this data set)
5 + 4 + 3 + 2 + 1 = 15
15 / 5 = 3
The mean of [5, 4, 3, 2, 1] is 3.