To understand how an object moves, we can treat it on its own, as an isolated object, and look only at the forces acting on it. A falling skydiver has two. Weight acts downwards and stays the same. Air resistance (drag) acts upwards and gets bigger as the speed increases.
At the start of the jump the speed is low, so air resistance is small and the resultant force is downwards. The skydiver accelerates. As the speed rises, air resistance increases, the resultant force decreases and the acceleration falls. Eventually air resistance equals weight, the resultant force is zero and the skydiver falls at a constant speed. This speed is the terminal velocity.
When the parachute opens, air resistance suddenly becomes greater than weight, so the skydiver slows down. As the speed drops, air resistance falls until it equals the weight again, and the skydiver falls at a smaller terminal velocity. The same idea applies to vehicles: a car reaches its top speed when the driving force is equal to the total resistive force, so the resultant force is zero and the speed is constant.
