The interquartile range is a measure af variability from statistics. It is the difference between the upper and lower quartiles, that is the range from the bottom to top 25\%. It is a more stable measure than the standard deviation, so better for many forms of visualisation or communicatio. However, it is easier to work mathematically with the standard deviation and variance, hence why they are commonly used.
Used in Chap. 7: page 103
Links:
alandix.com: interquartile range