The five number summary describes the distribution of the set of data by breaking up the data into 4 quarters and the numbers determine the boundaries of these borders.

Written in ascending order the five number summary consists of the:

Minimum, First Quartile $Q_1$, Median, Third Quartile $Q_3$, Maximum

The minimum is the smallest value in the data set and the maximum the largest number. The median is if found by determining the middle number that divides the data set into two equal parts. The first quartile is then the median of the first half of the data set and the third quartile is just the median of the second half.

Not only does these five numbers describe the distribution of the data but they can also help you construct a box plot.