An object is in free fall when the only force acting on it is its weight. Close to the Earth's surface, a freely falling object accelerates at 10 metres per second squared (10 m/s2). This means its speed goes up by 10 m/s every second.
A stone dropped from rest has a speed of 10 m/s after 1 second, 20 m/s after 2 seconds and 30 m/s after 3 seconds. The number 10 is the same as the gravitational field strength of 10 N/kg at the Earth's surface.
The acceleration does not depend on the mass. A heavy object has more weight, but it also has more mass to accelerate, so the effects cancel out. Without air resistance, a heavy ball and a light ball released together land at the same time. In air, a feather falls slowly because air resistance on it is large compared with its tiny weight, so it is not in free fall.