Free fall refers to the downward motion of an object without any resistance. Any object thrown upwards or dropped from a height is pulled towards the Earth's surface due to Earth's gravity. Such objects experience a constant acceleration, known as acceleration due to gravity (denoted by g), which has a numerical value of 9.81 m/s^2. Objects in free fall will not experience any other force and will fall with constant acceleration.
However, this is an idealized scenario. In reality, air offers resistance to any object falling downwards and this results in object achieving a constant velocity, known as the terminal velocity. This is a result of gravity pulling object downwards and air resistance opposing this motion. More the air density, higher will be this resistance. Air density is a function of altitude (apart from temperature). The higher the object, lesser will be the air density and hence lesser air resistance. Thus, the objects that are dropping from high altitudes are in (nearly) free fall, while those close to Earth's surface will have significant air resistance and hence will not be in free fall.
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