In the aerospace context, AGL stands for Above Ground Level, it is a term used to describe the altitude of an aircraft or other object above the ground or terrain. It is typically measured in feet or meters.
AGL is important because it can affect the aircraft's performance, such as its ability to take off or land, and it can also affect its visibility and communication with air traffic control. It is also used to make sure the aircraft is flying safely and legally, for example maintaining a safe separation from obstacles and other aircraft.
For example, an aircraft flying at an altitude of 10,000 feet above sea level (ASL) would be at an AGL of 8,000 feet if the terrain below it is 2,000 feet above sea level. Another example would be a helicopter flying at AGL of 500ft while it's hovering above a tall building, allowing it to perform maintenance or construction work.