Baseline
The invisible line letters sit on; tails such as g and p hang below it.
See it in Text flow. Turn on Show baseline.
In plain words
Letters in a line of text all rest on the same invisible line. That line is the baseline. Most letters sit on it, while tails like the ones on g, p and y dip below. Lining up text from different fonts by their baselines makes them look tidy. Line height is measured from one baseline to the next.