In a given data set two important parameter are the center of the data and the spread of the data. The center can be easily calculated based on the median mode etc. range is a measure of the spread of the data.
In general arithmetic the range is the difference between the largest and the smallest dat. For Example for a data set 2, 5, 7, 9, 15 the range is (15-2) that is 13. However in descriptive statistics the range is defined as the size of the smallest interval of data that contains all the data giving the true indication of the statistical dispersion.
However range is a very crude measurement technique. It is based on just two data and hence is very badly effected by outliers.
For Example say a data set has all values between 2 to 10 so the range would be 8, however the addition of a single data say 56 would increase the range to 54. The range is also used to measure the standard deviation. In order to overcome the effect of range of a data, box plots are constructed and interquartile range is measured. The interquartile range tells us about the spread of the middle 50% data.