A box plot gives you a simple overview of five of the most important values in statistics: Minimum value, first quartile ($Q_1$), median, third quartile ($Q_3$) and maximum value. Between the first and third quartiles you draw a box. The opposite sides represent the quartiles. You draw a line through the middle of the box, representing the median. The length of the box is the interquartile range. You draw a line down from the bottom of the box to the minimum value and a line up from the top of the box to the maximum value. A box plot is therefore a visualization of the dispersion in the data.