In histogram, mean and median describes the central tendency of a large data set.  When the data set is large, it is useful in representing the entire data set with a single value that describes the average or the middle of the entire set. In histogram, that single value is known as the central tendency and mean and median are the various ways of describing it. In order to understand the mean value, all the values of the data set needs to be added up and then divided by the number of values that have been added. The range, that is the difference between the smallest value and the highest value in the data set, shows how well the central tendency is representing the data.  For example: In a data centre rack, 3 servers are consuming 90 watts, 120 watts, 150 watt of power respectively. The mean power use is (90+120+150)/3 servers= b, which is the calculated mean per server. 

In the data centre, means and medians are tracked over time in order to spot trends.  The median of a group of numbers is the middle number, when the numbers are in order of magnitude. For example:  if the number set is 4, 1, 6, 2, 6, 7 and 8 the median is 6 as 6 is the middle number when the number are arranged in the order of magnitude.