That's a picture of a scrum. When a ball goes "off sides" the scrum occurs. "off sides" is any time a ball is sent forward by touching it with your hands, on purpose or accidentally. That mass of people is formed (it's very carefully formed, your head and neck are in bizzarre positions) and the scrum is engaged (you lock your heads together and apply pressure)....then the scrumhalf carefully puts the ball in the middle. The hookers try to hook the ball with their foot and send it back to their own team....the scrum players push forward to give their hooker the advantage.
It's all about control.
If the ball goes out of bounds, you have a line out, which looks like this: