Why does a skydiver go through two different terminal velocities?

When the skydiver jumps out, they are instantaneously at free fall (accelerate at 9.81 m/s). After this, a drag force from the displacement of air is felt. The drag force is proportional to the velocity squared. As the skydiver's velocity continues to increase, this drag force gets larger and larger, and the acceleration decreases. After a certain amount of time, the force from air resistance is equal to the force due to gravity. At this point, there is no resultant force and therefore no acceleration and the first terminal velocity is reached.
After the skydiver opens their parachute, the surface area exposed to the air is greatly increased. The drag force is then much larger than the gravitational force, so the skydiver decelerates. Decrease in velocity decreases air resistance, and a second much slower terminal velocity is reached once the forces are again equal.