The altitude Status widget shows you a summary on the current altitudes of the UAVs in the swarm:
The label under the icon shows the current altitude source.
The top row shows the maximum altitude.
The bottom row shows the minimum altitude.
UAVs can measure altitude relative to home (AHL), relative to ground (AGL), relative to mean sea level (AMSL) or in a local Z coordinate (XYZ). If you move the cursor over the widget, its tooltip will detail you which altitude source gets displayed actually and which will be displayed by pressing the widget icon.