Constant acceleration is a change in velocity that doesn't vary over a given length of time. If a car increases its velocity by 20 mph over the course of a minute, then increases by another 20 mph the next minute, its average acceleration is a constant 20 mph per minute.
The rate of acceleration due to the Earth's gravity is a constant 9.8 meters per second per second. An object in free fall continues to accelerate at a constant rate; but as a falling object's velocity increases, so does the force of drag caused by air resistance. When the force of drag equals the Earth's gravitational pull, the acceleration becomes zero, and the object continues to fall at a constant speed. Physicists call this final speed the terminal velocity.