The terminal velocity of an object falling towards the ground is the speed at which the gravitational force pulling it downwards is equal and opposite to the drag from the atmosphere (also called air resistance) pushing it upwards. At this speed, the object ceases to accelerate downwards and begins to fall at constant speed.
For example, the terminal velocity of a skydiver in a normal free-fall position with a closed parachute is about 120 mi/h (195 km/h). This speed increases to about 200 mi/h (320 km/h) if the skydiver pulls in his limbs.
The reason objects reach a terminal velocity is because the drag force depends on the speed. At low speeds the drag is much less than the gravitational force and so the object accelerates. As it speeds up the drag increases, until eventually it equals the weight. Drag also depends on the cross-sectional area. This is why things with a large surface area such as parachutes and feathers have a lower terminal velocity than small dense objects like bricks and cannon balls.
