A weighted mean is an average that takes into account the weight (or frequency) of each value.

For example, given the data 3 3 5 5 5 5 12 13 13 13, the mean can be calculated as follows: (3 + 3 + 5 + 5 + 5 + 5 + 12 + 13 + 13 + 13) / 10 = 77 / 10 = 7.7

Notice that 3 appears 2 times, 5 appears 4 times, 12 appears 1 time, and 13 appears 3 times, so this data can also be presented as a table:

The total can be found as 3(2) + 5(4) + 12(1) + 13(3) = 6 + 20 + 12 + 39 = 77; the total frequency is 10; the mean is 77 / 10 = 7.7. 

So, the weighted mean is [the sum of (each value times its frequency)] / (total frequency).