Short answer
The vertical distance from the front waistband to the front crotch seam. Correct rise is why bespoke trousers feel effortless where ready-to-wear pulls.
In bespoke, rise is not a style preference but a decision made with your tailor — recorded in your Fit Passport so the next commission starts from the same understanding. Ask about it at your fitting; the honest answer includes whether it serves your line or is merely tradition.