The indicator shows the first, second and third quartiles of the sample. Quartiles is a set of quintiles for p=0.25, 0.5, 0.75. Their estimates (quartiles of the empirical distribution) are the values that divide a data sample into four groups containing (if possible) the same number of observations.
When talking about quartiles we usually mean upper q3 and lower q1 quartiles; the second quartile is the median q2.
Lower q1 quartile is the value, below which a quarter of data is located in an ordered set, while upper q3 quartile is a value, above which a quarter of data is located in an ordered set.