The Trimmed Mean (also known as the truncated mean) is a measure of mean that indicates the central tendancy of a set of values.
The Trimmed Mean is calculated by discarding some of the values at either end of the range of values, before calculating the arithmetic mean of the remaining values. This prevents the calculated mean being skewed by extreme values (also known as outliers).
For further information on the Trimmed Mean, see the Wikipedia Truncated Mean pageThe Excel TRIMMEAN function calculates the trimmed mean (or truncated mean) of a supplied set of values.
The syntax of the function is:
where the function arguments are:
array    An array of numeric values, for which you want to calculate the trimmed mean. 
percent    The percentage of values that you want to be discarded from the supplied array. 
Note that the specified percent value is the total percentage of values to be excluded from the calculation. This percentage is divided by two, to get the number of values that are removed from each end of the range.
It should also be noted that, when Excel is calculating how many values to discard from the supplied array of values, the calculated percentage is rounded down to the nearest multiple of 2. For example, if you want to calculate the trimmed mean of an array of 10 values, then:
Cells B1B3 of the spreadsheet below show 3 examples of the Excel Trimmean Function, all of which are used to calculate the trimmed mean of the values in cells A1A10, for different percent values.
Formulas:

Results:

Note that, in cell B1 of the spreadsheet above, the supplied percent argument is 15%. As there are 10 values in the supplied array, the number of values to be ignored is 1.5 rounded down to the nearest multiple of 2 which is zero.
Further examples of the Excel Trimmean function are provided on the Microsoft Office website.
If you get an error from the Excel Trimmean function this is likely to be one of the following:
#NUM!   
Occurs if either:

#VALUE!    Occurs if the supplied percent argument is nonnumeric. 