When you let go of a rock, it falls, gaining speed at the rate of 9.8 meters per second every second. That’s the influence of the earth’s gravity. If you toss it straight up, it loses speed at the rate of 9.8 meters per second every second. Both these statements can be condensed into the statement that the rock’s downward velocity increases at the rate of 9.8 meters per second every second. If there are no influences but gravity on an object, then that object gains downward velocity at the rate of 9.8 meters per second every second, and we say that the object is in freefall.

Basic ballistics is all about freefall – once a cannonball has left the cannon, the only force acting on it is gravity (again, neglecting air resistance).