Above Ground Level, or AGL, describes the literal height above the ground over which you’re flying.
Mean Sea Level, or MSL, is your true altitude or elevation. It’s the average height above standard sea level where the atmospheric pressure is measured in order to calibrate altitude.
On a Sectional Chart, all of the numbers you see that denote altitudes are denoted in MSL, unless they are in parentheses. If you see a number in parentheses, that denotes AGL.
Interestingly enough, the MSL calculation is derived from observations of tides and seasonal variations over a 19-year period to arrive at the average MSL. A plane that flies at 10,000 feet MSL and stays level registers as flying at 10,000 feet MSL — no matter the terrain changes below the pilot.
Pilots use altimeters, which measure the AGL, when the aircraft is flying at relatively low heights landing at an airport. But as the plane increases in altitude, the altimeter reading becomes less accurate. Once the plane reaches transition altitude, the aircraft uses the MSL along with air pressure readings to maintain level flight.