Consider a parachutist who jumps from a perfectly good airplane.
What forces act on this parachutist? Certainly, there is a force of gravity, the weight W,
But there is also a force of air resistance, F_{air.}
This force of air resistance increases as the parachutist falls faster and faster,
At some speed, the upward force of air resistand F_{air} will just equal or balance the downward force of gravity, the weight W. At that velocity the net force will be zero and the parachutist will have zero acceleration. That means the parachutist's speed will remain constant. We call this speed the terminal speed or the terminal velocity.
The force of air resistance is very important for a sky diver!
Sometime, tho', the force of gravity may not be important  it may be far less than the force of gravity. If we drop a golf ball from shoulder height, the force of air resistance on it is far, far smaller than the force of gravity. We can approximate cases like this by free fall. We would have to measure things very, very carefully to find any deviation from our approximation of free fall.
