The geoid is the shape that the ocean would take under the influence of the gravity and rotation of Earth alone. The geoid is smooth but irregular because Earth’s density and surface are uneven. ‘Height above sea level’ is measured relative to the geoid.
Some flat-Earthers claim that measuring height above sea level is not possible if the Earth is spherical. They are wrong. Height/altitude/elevation is usually measured relative to the geoid, which is approximately a sphere. The geoid is a more accurate model depicting the shape of the Earth, compared to the spherical & ellipsoid model.