The observation which occurs maximum number of times in a given set of observation is called is called mode.
In other words, the observation with highest frequency is called mode.
Eg. 2,2,3,5,6,4,7,2,2,3,6,2,6,2,8,2,2,3,8,9,2,2,2,3,7
In the above set 2 occurs maximum number of times, so mode is 2.