Ground reaction force is the force produced by the ground upward against your feet when standing. Otherwise you'd fall through!
So in this study they had kids jump downward from a 61 cm box. The force at ground contact was 8 times their body weight. 90 pound kid = 720 pounds of force. The cool thing about the study is that is shows the force absorption capabilities of the study group and that this form of training under appropriate supervision is beneficial. While there is no form of training that is 100% safe, the fear of serious injury (the ol' growth plate concerns) is most likely exaggerated. Increased bone mineral content is especially an issue for girls as the density of their bones gained at a younger age carries over quite well to adulthood.
Bill