The vertical space between lines of type is called leading. Leading is measured from the baseline of one line of text to the baseline of the line above it. Baseline is the invisible line on which most letters—that is, those without descenders—sit.
The default auto-leading option sets the leading at 120% of the type size (for example, 12‑point leading for 10‑point type). When auto-leading is in use, InDesign displays the leading value in parentheses in the Leading menu of the Character panel.