As the skydiver is falling, she is pulled down by the force of gravity, which tends to accelerate her towards the earth, while at the same time the drag force Fd opposes this motion. When she first jumps out of the plane, her speed downward is not very big, and so the drag force is small. As gravity accelerates her downward, her speed increases, and so does the size of the drag force. As the drag force increases, it opposes the force of gravity, decreasing the size of the acceleration. At some point, the drag force will increase to be the same size as the force of gravity. At that point, the skydiver will stop accelerating and hit a constant velocity, known as the terminal velocity.
(b) When the parachute is opened, the b coefficient in the drag increases, and the drag force will be higher than the weight force. This will cause an upward acceleration that will have the effect of reducing the downward velocity. As the velocity decreases, the drag force will decrease, until the drag force once again balances the weight force at a new, lower terminal velocity, hopefully leading a safe landing!