The “box” part of a boxplot outlines the lower and upper quartiles. Inside the box is a line that indicates the median value. There are lines that extend outside the box—known as the whiskers—to depict the range of values in a given dataset. If there are outliers, then individual dots in line with the whiskers are plotted to denote the extreme values.
Click through for a depiction of the plot as well as several alternative depictions which can include more information at the cost of added complexity.