Initially, the upwards air resistance is very small because the skydiver isn't falling very quickly
Therefore, there are unbalanced forces on the skydiver initially
As the skydiver speeds up, air resistance increases, eventually growing large enough to balance the downwards weight force
Once air resistance equals weight, the forces are balanced
This means there is no longer any resultant force
Therefore, the skydiver's acceleration is zero - they now travel at a constant speed
This speed is called their terminal velocity
When the skydiver opens the parachute, the air resistance increases
This is due to the increased surface area of the parachute opening
The upward force of air resistance on the skydiver increases, slowing the acceleration of the skydivers fall
The skydiver decelerates
Eventually, the forces balance out again, and a new slower terminal velocity is reached
