Stack height measures the vertical distance from the center of the bottom bracket to the top of the head tube — basically, it determines how tall the front of the bike is relative to where you pedal. A higher stack puts you in a more upright, comfortable position, while a lower stack leans you forward into a more aerodynamic, aggressive stance.
For endurance road bikes, expect stack heights roughly 10–30mm taller than race bikes in the same size. If you're flexible and focused on speed, a lower stack works well. If you prefer long, comfortable rides or have limited flexibility, aim higher.
While a bike fitter can tweak your effective stack with spacers and stem changes, it's best to start with a frame that's close to ideal. Don't chase a low, pro-style position just for looks — ride what fits your body and goals.