When an object falls it accelerates due to its weight (the downward force of gravity acting on the objects mass). As it accelerates its velocity increases. The increase in velocity is accompanied by an increase in air resistance (drag). Eventually the air resistance acting upwards on the objects equals the weight acting downwards. The overall force on the object is balance or zero; it therefore cannot accelerate and continues to fall at constant velocity. This is referred to as the terminal velocity.
The animation below shows the terminal velocity of a skydiver: