Aviation states a flight level in two ways. AGL (Above Ground Level) is the height above the terrain directly below the aircraft — it changes as the ground rises and falls. AMSL (Above Mean Sea Level) is the absolute altitude above mean sea level — independent of what lies underneath.
For a drone pilot the distinction matters in practice: the 120 m open-category limit is measured from the closest point of the surface, so it is an AGL-type height, not a sea-level altitude. Flying over a hill, the permitted ceiling rises together with the terrain.