Altitude has many different references and definitions in aviation. The two main ones you need to know are:

AGL – AGL stands for “above ground level,” and it indicates an altitude using the ground as a reference. This is also known as absolute altitude.

MSL – MSL stands for “mean sea level“,” and it is your altitude above sea level. 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.