A box plot is a 5-number summary: min · Q1 · median · Q3 · max. Points farther than k·IQR from the box are flagged as outliers (Tukey's rule, k=1.5 by default).
Q1—
Median (Q2)—
Q3—
IQR—
Outliers—
1.5
Tukey's rule: lower fence = Q1 − k·IQR, upper fence = Q3 + k·IQR. Slide k down to flag more, up to flag fewer. k=1.5 is the default; k=3 marks only extreme outliers.