Initially, there is only one force acting on the skydiver, which is their weight. F=ma therefore mg=ma. The m's cancel and the skydiver falls with acceleration g. However as soon as they have a downwards velocity, drag acts upwards to oppose this motion. As the velocity increases, the drag increases and therefore the force acting downwards on the skydiver decreases. This continues until the drag force is equal to the weight of the skydiver. At this point there is no net force on the skydiver and since F=ma, this means that there is no acceleration and the skydiver is in freefall and has reached their terminal velocity